My 1 year old yorkie just got over a false pregnancy and although it sounds like something harmless and sweet,you should really go to the vet because it can cause illnesses like pyometra and mastitis which can both be fatal if not treated.Jessies boobies got very big and she even produced milk,ALOT of it,she was uncomfortable and they obviously hurt.This can cause mastitis which is not only painul and uncomfortable but it can be fatal.Squeeze one of your chihuahuas nipples gently and see if she is producing milk..you dont have to do it hard.Whilst Jess was having her false pregnancy the milk would just drip out.
Your vet will give you something to stop it straight away if she really is having a false pregnancy,the meds take about 15 days to work so you had better get there as soon as you can.
Here is some info on false pregnancies,I found this whilst Jess was having hers:
http://www.*********.com/mbbs2/forum...ighlightmode=1