the vet should be able to tell if she is pregnant if she is far enough along. If you do NOT want her spayed if she is pregnant, than just postpone it and wait. If you will get her spayed either way, then the sooner the better (while they are small)

She may just be scared because she is more timid. It takes some cats longer to adjust to a new environment and new furpals. Give her time and introduce slowly. My pesonal experience is females are harder to introduce than the easy going males (But that is only my opinion)

She could be eating a lot because she always had to fend for herself, and not knowing where her next meal would be coming from. I found that it took quite a long time for a few of my cats to stop eating like there was no tomorrow and it had nothing to do with pregnacy.

If you are prepared to responsibly find homes for each and every kitten, take care of them for the full 8 weeks and give them all the necessary medical care required to a mom and babies than you can allow her to have the babies. But sometimes abortion really is the best choice.

Hope some of this is a help to you. Good luck