Totally normal. Sometimes after having sex a few times my penis is sore as fuck. You just had to throw the "why" in there - I'm stumped. I would imagine it's because: your penis is flaccid most of the day right? Relatively little stress on the blood vessels and chambers there. Regular erections come and go through out the day, and then you masturbate. When you come you put tons of stress on the vessels/chambers by the hyper-influx of blood and PC fluttering. The next time you get an erection, it just picks up on whatever aggravation you caused your penis during ejaculation. But, it's all speculation from my end. Maybe someone else will fill us in.
By the way, does your name mean 7.5" At The Moment? I've been trying to figure it out for days...