From your description, it sounds like your kitty just loves to knead (this is a security behavior that goes back to when she was nursing from mom--the kittens will knead around the teat to help the milk come down). Some cats are big kneaders and others rarely or never do it. Sometimes, they seem to go into a trance and will even salivate while needing (again, going back to the nursing).
If your girl is spayed, she should not be exhibiting any sexual behavior, unless her spay was incomplete* or she is having a hormonal problem. If she is not spayed, this may be related to her aggressive kneading and she should be evaluated by her vet and spayed, asap. If she hasn't been to the vet since you found her, definitely make an appointment for a physical and the vet should be able to tell if she's been spayed or may have to do an x-ray and blood work (to check her hormone levels), if not 100% sure from the physical exam.
*An incomplete spay is when some ovarian tissue is accidentally left behind and is enough to release hormones making her go into heat. You likely would have seen symptoms of estrus or heat before now, if she was not spayed.
If she does this aggressive kneading on your legs/lap and her claws are sharp, make sure to have them trimmed, so she doesn't hurt you and you can also keep a blanket or towel on your lap/legs, when she kneads, so it hurts less.
If she's eating, drinking, eliminating, playing and acting as usual, that's great, but a vet visit, to investigate this new aggressive kneading would be advisable.
I hope all will be well with your furry girl and you found my answer helpful.
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