Firstly I highly recommend that you visit your GP and make sure that everything is in order, particularly if you have only started to feel the pain recently. There's probably nothing to be worried about but it wouldn't hurt to make sure there are no underlying medical issues.

If you've recently been checked out by a doctor and given a clean bill of health, then it may simply be that your partner's penis is butting against the cervix which isn't pleasant at all.

Normally, if you are fully aroused, your vaginal canal elongates sufficiently to accommodate a penis (even those that are 10 or more inches) so the cervix is generally well out of the way during sex. However, if your vaginal canal hasn't elongated enough then even a regular sized penis will end up causing pain.

Alternatively, you should examine what positions cause you the most pain. It could be that your hips are ending up in a position that exposes your cervix more than what is comfortable. Try a few different positions to see what works best for you and make sure you are aroused and well lubricated at all times.