You're too young to have "weak" stomach muscles, there is an underlying problem. I agree with Jaybay, see an OB-GYN. Have you seen a Gastroenterologist? If you google "weak stomach muscles" it does not have anything that pertains to someone like you who is young and never had babies. There is a condition gastroparesis but this bloats your actual stomach, not the area you're speaking of. I think your doctor just doesn't know what is wrong, and therefore it's IMPERATIVE that you speak to your parents and tell them not to accept this lame diagnosis. I think it's something with your reproductive organs. Please let us know what docs you have seen, and please emphasize to your parents that this is not a diagnosis, and they truly need to persue this until they have answers!!!!!
I'm really sorry to hear you're having such a terrible problem at such a young age. Is there a reason you don't get much exercise? I do agree with your doctor that strengthening your muscles will help you deal with it better, but won't cure the underlying problem. Same thing for the heat and ice - you're treating the symptoms but not the cause.
Have you ever been to a gynecologist? Since your issue is so low in the pelvis and that is where your reproductive organs are located, a gynecologist may be able to pin down the problem.