So my chihuahua came into season for the first time last month. We have a male DDB. We kept them separate but there was one occasion they managed to get together when we were out.
I didn’t think much of it as thought it was impossible for anything to happen, but since my chihuahua finished her season her nipples seem to be getting bigger.
Is it possible she could be pregnant? I’m terrified. This was about three weeks ago.

this is likely to be a natural part of a bitch whom has finished her season as she came in a month ago.

I am sure you realise you should not let your bitch have a litter by your male, it would kill her if she became pregnant, due to the shear difference in size.

For you own peace of mind & for your bitches sake you need to get her to a vet!

It could be a phantom pregnancy (normal) or it could be a real one, given the size difference it's unlikely, but not impossible. She can be ultrasound scanned for pregnancy from 28 days after mating, which would be sensible. If the scan shows that she is pregnant it would be sensible to spay her immediately; although spaying when the hormones are active is usually a bad mistake and can have several longterm side-effects, they are insignificant compared to the massive risk to the bitch's life of producing even one pup of this breed combination.

Thank you. Yes I definitely would not let her have the litter. I’d feel awful if she is pregnant and horrible having to terminate, but will have to do it for her safety.

Never say never with nature BUT I'd suggest given the size difference alone, this would be unlikely to have happened (mating - pregnancy).   For sure she'll never carry to term even if the worst has happened.   If she were mine, I'd not wait for the scanning time but get her in for a spay NOW, telling the vet she might be pregnant.   It sounds as if spaying should be on the agenda for her, in any case, if you have an entire male in there.   Kinder to him, if not her!

> For you own peace of mind & for your bitches sake you need to get her to a vet!

30 - 35 days from when you left them together get a scan done (the later timing is netter for a conclusive scan). 

If it turns out she is pregnant a gravid spay can be conducted or Alizin injection can still be given, but this will cause an abortion rather than absorption of the foetuses, but safer alternative than allowing a pregnancy to continue.

I would have had the Alizin injection 3 weeks ago.

Have you taken her to the vet yet?  The sooner the better for all concerned