first off welcome, youll fit right in lol.
and secondly, you get a medal for rescuing this wonderfull little girl! she will bring alot of love and jy to your life.
Your right, the woman you got the chi off lied, a female doesnt come out of heat to go back in, she was either not in heat before and came in now or was in her heat and she just dindt want to tell you.
and dont feel so bad for this happening, you wernt to know.
now that being said, im pro choice in life, but dont let that fool anyone into thinking thats why im going to say what im going to say. what i say is from experience.
your obviously unexperienced when it comes to breeding chihuahuas, and thats not a bad thing
the biggest thing to rember is that this female is probably already weakend from her previous litters, the more pups a female has the more weak she gets, especially when there one after anohter... this pregnancy could realy weaken her, to a point where she may not have the resources after birth to suply her self and her puppies with the nutrients needed to stay healthy.
also this breed is extreemly prone ot c-sections, while this femalesprobably freewhelped all her life (probably given very little choice) she could easily hav eproblems with delivery too, you could lose the entire litter and the female.
dogs systems dont work quite like ours, the easiest way to explain it is that there is a delay between the males "donation" and the fertilization, so IF shes pregnant any pups would not yet be what we think of as pups.
most breedings dont take on the first mating, (its why most breeders mate their female 3 times to a male) but it IS a possibility.
for the saftey of your new baby, (and your sanity lol) i would personally go ahead with the spay, make sure to tell the vet that theres a possibilty shes pregnant. the vet might suggest waiting untill after the heat is over, but i personally think a spay (with possible abortion if needed) is the SAFEST for your chi