Also, were the blotches there before intercourse?
I think that it is important that we establish whether or not you were using protection.
Adam
Did you use some kind of condom or lubricant? Is it possible it's an allergic reaction? Allergies to those sorts of things usually come with swelling, pain, and itching but not all the time. It may be nothing more than a rash so make sure you get it looked at and not just take the blood STD test.