tense before sex

Something weird has been happening lately when I'm about to have sex, I have been quite tight and as if I'm tense inside my vagina so it becomes painful for my boyfriend and he almost can't get it inside me, although I have been having sex with him for almost 2-3 years now, however after it finally goes in I'm no longer tense ? Why is this happening ... Is it normal ? Is their something wrong with me ? Please help