My girl had a litter of puppies and I kept one of the girls who is now 6 months old.

At no time did mum show any signs of protectiveness in any situation, whether it was strangers coming to few pups or going out on walks with pup I kept.

Recently however when on walks with pup she has began to randomly bark at people, always the same scenario, others dog owners stroke puppy and go to stroke mum, at this point she sometimes barks at people. Both dogs are off the lead and it is in no way to protect her puppy. (puppy is happily bouncing around with other dogs etc - mum never been that interested in other dogs) Often she will go to the person in her normal friendly way and then give off a grumpy bark. This isn't everyone she meets, just happened to 1 out of 4 people yesterday. It seems like an almost involuntary bark that isn't really aggressive more grumpy. She has done it to complete strangers but also to dog walkers we see daily who she gladly runs over to say hello/get a treat from.

I'm assuming this is a hormonal issue as she is coming in heat soon again but just wondered if anyone had similar experiences or suggestions?

What's she like when she is walked without the puppy? I would consider doing separate walks for now and see if there is any difference in her behaviour.

sorry I should've have said. She's the same walking alone. Even done it at home to my brother-in-law who she see's regularly. Puppy is very playful and excited with other dogs and people and mum isn't remotely concerned when pup plays with dogs or people. The barking is when people have gone to stroke her, even if she has actually walked over to them herself. She has done a grumpy bark, she has often gone back to person and let them stroke her.
before and since birth mum is very chilled out on walks, will say hello to other dogs but not that bothered about playing with them, always likes a fuss from people and happy to be stroked etc, she knows the ball thrower comes out for her at the end of the walk. ball and food is all she's bothered about. now she's just started the random grumpy barking.

Is it possible she has a sore spot on her body and if theu touch that she barks ? Maybe a vet check?

I would get a full vet check done including eyesight.

Agree.  Full vet check, including joints, spine (incl. neck), eyes, ears and teeth.  It sounds like people are stroking her and catching something.

I'd also check her over for any little lumps or bumps - years ago I went to stroke my new oldest, missed her head and touched her face and she yipped.  I had a look and she had a lump on her gum.  It was a highly aggressive cancer.  She showed no other symptoms.

I was going to suggest that even if she doesn't appear to be doing this, perhaps she is protecting the puppy.   I thought this because, idiot me, I saw a pretty puppy tied up with an older dog outside a local supermarket and bent to pet the puppy.   The older dog lunged at my face and bit me.  It was my fault but it scared me.  It was quite a bite and I came home and washed it etc. but on looking, decided to go to our local A & E where they checked it, gave me a course of a/bs and asked about my last Tet. shot.  They didn't feel it needed stitches which was good as there's no scar left.

However, if she's doing this without the puppy around when walking, perhaps this has to do with her coming into season soon - not that any of mine has ever been grumpy around that time.  If it's too close to this season, you may be wise to think about getting her spayed after this, and perhaps as suggested, have your vet check her over now in case she does have something hurting.

yeah she's got her booster jab soon so will get her checked over. she still is very keen on being stroked by me and everyone though, the bark doesn't happen to everyone consistently perhaps 2 or 3 out of 10 people which made me think it was more related to heat season than another physical pain she is consciously defensive of. always before any contact as well. strange one - thanks for the replies!

If she is defensive as u describe i would think she is expecting contact to hurt in some way.  It would be worth a check sooner than later imo

oh no sorry I meant it doesn't appear to be consciously defensive - she doesn't do it to most people just occasionally and seems almost involuntary and quite random when she does do it.(I think if she had an injury she'd do it to most people or everyone) literally all her other behaviour is the same - friendly and affectionate but just these new barks.

she's booked in next week so will see what vet says

That's why I'm wondering if something is hurting - that random factor.  Individual people catching the sore bit accidentally.

oh yeah, the bark comes before any contact just when people have moved to stroke her. She has then gone back and allowed them to stroke her (sort of apologising for barking almost) She has never barked after any stroke or contact. Very random!

Could it be her eyesight? She may not see these people until they are suddenly in her vision and this is worrying her? Certain angles coming in or light outside?

Defo a full vet check if this is not usual for her before any previous seasons