I dont think you have anything to worry about but am surprised your Dr did not pick up on the fact that you worked out and that it simply could be where you sweat and skin creases a little you don't have to be defined in the stomach to get this you can be very flabby or muscularly defined but as the muscle develops and you perspire then the defintion which is taking place secrets the sweat and SALT into the areas causing redness and sometimes itching. People with IBS may have a similar red mark due to erm... 'sweaty bottom' in the crease and along the crease of there bottom cheeks (This is the only link I can think of). This is not a jest I can assure you, but I think your problem maybe to do with excessive sweat or even heat (like heat generated in a heavy workout) that causes the redness and rash. The best ting to do with your Dr is to perhaps mention this and if you get an unsatisfactory reply seek a further Proffesional opinion. The only reason this sprang to mind is that my friends and I used to work out ages ago and he used to come out in a similar sounding rash. Perhaps try some moisturising (sp?) cream and see how you go ?