You really should not have any form of penetrative sex while you have a tampon in your vagina. Not only could it bump your cervix rather forcefully, it could also cause abrassions to your vaginal walls. And really, it wouldn't be particularly enjoyable for either of you.
If you want to have sex during menstruation, take the tampon out first. But be doubly careful to use condoms, because you *can* get pregnant, and your partner will also be exposed to your blood.
Some women have found that tampon alternatives such as the Keeper or Instead (both are cup-like devices that catch your menstrual blood -- run a search, we've had several topics about them on the boards) are good for having sex during menstruation if you want to avoid the blood. I've not tried either, but I've heard good things about both, and it might be worth a shot.
"The only unnatural sexual act is that which you cannot perform."
-- Alfred Kinsey