I have a young bitch - she is coming up 20 months. I posted before that she was very late coming into season ( just after 18 months) About now we would be thinking of puppies if she had been mated if you see what I mean. I have just discovered that around some not all of her mammary glands there is definite lumpiness. I don't remember this with any of my other girls in the same way and am actually quite worried. Just had her on the grooming table and had a feel because she had  sort of been nibbling herself underneath. The nipples themselves are still quite tiny so no enlargement there.
Anyone experienced this and if so especially in one so young. I think I will book her into the vets tomorrow for a check over.

My 19 month girl has had two mammary lumps in the past few months.  They have been in two different nipples and felt very big.  Once I was handling the lump to feel how big it was and some liquid(fairly clear) came out of the nipple.  My girl was spayed when she was a year old after having her first season.  The first lump we felt worried me to death and I took her straight to the vet, who wasn't sure what it could be.  She had a week of antibiotics just to be safe and we returned a week later for a follow up and the lump had gone.  A month ago she had another one which we left and it went down within a week.  The vet has put it down to hormonal flare-ups.  Your case may be different as your girl hasn't been spayed.  I just thought it sounded familiar when I read your post.