Fist sized knot pops up in uterus after sex

I am 32 years old have 3 kids had tubes tied 7 yrs ago n uterine ablation due to heavy painfull bleeding 5 yrs ago. Deep intercourse is painfull n had bright red bleeding after sex in oct for couple weeks now after sex or orgasm my uterus pops up with hard fist to grapefruit sized knot alot like when I was pregnant. I am not pregnant still having periods though light only 1 or 2 days sometimes n irregular but I look 5 months pregnant!! People keep asking when I'm due!! I've alwYs been thin 5'4" n usually 120lbs in past year up to 140 all in belly n boobs just like I'm pregnant! Serious dieting n exercise have gotten me to 133 but I look so pregnant and there's something in there freaking me out! Please help!