The test line has to be as dark or darker than the control to be considered positive on an OPK.
That being said, LH and HCG are very similar in composition. OPK's can pick up HCG and show as a positive. It's also possible that you're getting close to O if you're about a week after your "AF". (Even if it wasn't very heavy) Your symptoms do sound promising though, so I'd take a HPT just in case