I would worm her regardless of whether you believe she is pregnant or not as intestinal parasites are no good for her. As a feral she most likely has them as I'm sure she's been eating wildlife, etc. to survive. Is she urinating enough? If she has a urinary blockage of any type her belly could swell from backed up urine. If you have lots of time and patience and you can just sit by her and watch her tummy you may see kittens move. If not, of course it still doesn't mean she's NOT pregnant. Ultrasound would be the best way to find out. Also would tell you how close to birth. 45 days or older you will see skeletons. I know she won't let you squeeze her nipples but she could be producing milk. She sure is a beauty!