She went outside yesterday and was gone for about 6 hours - a record for her. When I found her she was under a car hanging out with another cat, I am not sure if that cat was male or female.
My question is this, if she mated with that cat - would she still be doing the signs of a cat in heat or would that have stopped after mating? If she did mate, how long will it be before I know if she is pregnant? Can she be spayed when she is pregnant? And will that cause any harm to the unborn kittens?
My question is this, if she mated with that cat - would she still be doing the signs of a cat in heat or would that have stopped after mating?
Whether or not she mates, a female cat's estrus cycle will last approximate seven days, and sometimes longer. She may mate with multiple toms during that period of time, and later give birth to kittens from each of those toms. (This is called "superfecundation.")
If she did mate, how long will it be before I know if she is pregnant?
You will probably not observe any early signs of pregnancy. Around three weeks, her nipples may become pinker and/or more prominent. However, I would not wait that long, but would take her to the vet immediately.
Can she be spayed when she is pregnant?
Yes, she can be spayed while pregnant, but it may cost more than in her regular state. (It might also cost more to have her spayed while in heat.) However, in my opinion, the long-term health of your cat is more important than the costs involved in spaying. Spaying will help prevent a number of potential physical problems, as well as those hazards inherent in pregnancy itself. (Please read "Reasons to Spay & Neuter Cats.")
And will that cause any harm to the unborn kittens?
Spaying during pregnancy involves aborting the fetuses, since the uterus is removed. It is a serious and personal decision, one that people involved in cat rescue are faced with almost constantly, especially during "kitten season." I have covered those considerations in my article, "Should a Rescued Pregnant Cat Be Spayed?. It is safer to spay a pregnant cat when she is still in her first trimester. Since a cat's normal gestation period is nine weeks, that would be within the first three weeks. (That is the reason I advised above not to wait that long.)
Having said all of the above, I urge you to take your kitty to the vet as soon as possible, have her checked out, then make an appointment to get her spayed. She will be a happier, healthier cat, and you will have the comfort of knowing you have not only done your best for her personally, but you have taken a role in helping prevent our burgeoning cat overpopulation problems.