Cat acting strange!! HELP!?

About 3 years ago, our neighbor shot our cat not once, but twice. She had three holes because she must have been laying down so it went through her leg and then through her stomach, well she cleaned it up and there was no blood when we found out and it took her a long time to heal up. We couldn't take her to the vet at the time as we live in a small town and the vet works in another town too so we let her do her own thing and keep her away from the dogs for awhile. After she healed she was spaded so she couldn't have anymore babies so she gained quite a bit of weight.. I'm not sure what she weighs now but it's gotta be well over 30 lbs. I've noticed for about a week and a half now that she has had poop on her butt, all i thought to myself was that she's just getting old (going on 9 yrs) and she is over weight so she must not be able to reach down there to clean herself. Today i was walking into the hallway to use the restroom and she was in like a pooping position shaking facing the wall, I touched her and she shook worse so i kind of helped her to lay down and i pushed on her stomach not really hard and i heard this squeak sound like when you blow up a balloon and let it go. It was her fart and so i sat there with her kind of worried for 20 mins. and she was still laying in the position i left her and you could see the pain in her face evertime she lifted up her bum to let another one go.... i don't know if it has something to do with the bullets if they are still in her from years ago or what.. I'm just scared something is wrong with her because i've had her since i was like 7 or 8 years old because my other cat (we no longer have got ran over) was her mom and she had kittens like 2 days before my birthday and i asked if i could keep one. haha. If i have to.. I can ask my mom to bring her to the vet but maybe one of you have an answer :)