Hi georgie,
Painful sex is quite common, and as melons has said it can be for a variety of reasons. I dont think infection is an issue unless you have an accompanying discharge. Answers to the following may help-
Are you sexually experienced?
Do you feel 'wet' down below before penetration?
Is your boyfriend particularly well endowed?
What positions are you using, and are some more painful than others?
I often experience a bit of pain, mainly due to my boyfriend being quite 'big' and getting carried away lol!! Your last answer probably hits the nail on the head, you are maybe too tense. If the above questions dont lead you to think it could be related to any of these issues then I would follow melons advice and get checked out, maybe by your practice nurse in the first instance if you feel more comfortable with that.