I agree with Eva. Book an appt. for a pap and overall check-up and speak to your doctor about your concerns. It may be a medical reason: sex shouldn't hurt. And, I'd like to clear the myth about exercise and 'tightness' being a factor here. It's highly doubtful. Since you have been having sex for a while, and if you use proper foreplay/lubrication...there is no rush: if it hurts, stop! slow down!....'tightness' should not cause pain. Vaginas are made for penis'.

Exercise is good, but shouldn't cause troubles in bed...in fact, should be the reverse.
It's usually a matter of: Poor techniques in the sack, rushing/over-eager partners, not enough foreplay for proper natural lubrication, or not using enough bottled lube. OR
It is a medical situation. Hymen related. An infection. Lots of things....
This is interesting too...don't know if it could be in your case?
Title[/URL]
Vaginismus
Oh, and it may also be pyschological. It may be that you are tensing up, not relaxed enough...and rushing the process yourself. I can not speculate, but this seems quite common with women (and I guess men too). <insert>