I am not a breeder and never will be. my whippet pup aged 10 months is on day 19 of her first season- counting the first day of blood as day one.  from day 22 are we definatly safe regarding pregnancy risk? she has ceased bleeding but is still rather swollen around vulva and nipples are enlarged.  I am itching to take a decent walk over local golf course and there will be loose dogs there.   I always believed season was 21 days but looking online some sources are saying give a full 4 weeks to be safe.

I've always worked on a full month before I consider my bitch safe . Possibly  a bit ott but I would.rather be safe than sorry.

My stud dog mated a bitch on her 21 day and the bitch had a large litter.

thank you both. another week it is then!  I really do not want to take unnecessary risks.

A season can vary for bitch to bitch & even season to season.
I had a whippet whose first season was 5 weeks! So yes most say 4 weeks... I also know many years ago a whippet bitch was mated on day 21 of season & went on to have 9 puppies.
Swelling can take a little while to go down, so that is perfectly normal.

We have mated bitches on day 23 to 26 and have had for the breed large litters of 5 to 7 puppies it depends on the individual, one stud dog owner was saying even though she was standing it was to late, they mated on days 13 and 15 had for decades.  Yes we had puppies they bought second pick we both were looking at the same two girls and the one we didn't want they did we were nit picking on them,
.

But in answer to your question is yes she is still in danger we walked our in season girls on flexi leads having drove to a walk five minutes away so leaving on scent trail home.

we are just on lead  pavement walking nearby on quiet roads- and avoiding areas where folk have off lead dogs.  the social distancing due to virus at least does help with this as everyone is crossing the road if sees anyone coming the other way.  I think most people round here neuter their dogs so not seeing much of a reaction from other peoples dogs and she is small enough to pick up if really comes to it.

I had no intention of mating one of my bitches, and thought that her season was over.  She stood for a dog on day 28 and had a very large litter for the breed.

I was 'lucky' to have males and females so I always knew when it was over!!   I always gave mine a bath after the 21 days too.  As you don't have a male there (?) I'd give it another week before you let her off, if you do!!

After the first season, the vulva and teats are usually more obvious than when a maiden puppy.... from experience.   It's interesting that she came in when she did.   My vet asked me to let my Whippet have one season before being spayed so I held off, making arrangements for her to be out of here when she did come in (we had an elderly and sick male Basset at the time although he'd not 'do the job' - he would be upset tho).   As it happened by 11 months there was absolutely no sign of her coming in so I asked him to go ahead and do the spay.   I asked her breeder when her mum and half-sisters started and she said not until around 15 months!!

I expected older too.  but its spring, day length  and stronger sun I think can influence. I suspect had she been 10 months in say November she would not have started right now. just my theory.    no other dogs here to get a reaction off , will give it a week before relax a bit lol

> I expected older too

Again this can vary, I have known some whippets come in season before 6 months old. Of the 5 bitches I have here, the 14 year old was 9.5 months when she came in season. Her 3 daughters were 11, 13 & 14 months old & the daughter that came in at 11 months, her daughter was also 11 months. Time of year had nothing to do with it.