sorry i didnt mean to jump to conclusions, but alot of over weight people dont have alot of room when they are having sex, thats why i just mentioned that, maybe turn slowly down, or if you really like it the 'hardcore" style, try doggie.
I'm 108 pounds and he is 140 pounds, so, I think loosing weight would be a bad idea. Don't assume things (;
try tolcon power also known as baby powder, usually does the trick, sounds like some sort of rash, maybe next time, dont have sex as rough, or one of you's needs to loose weight, so theres more room
:)