Pain during sex?

Hello Girls, I have a intimate question to ask.. 
Since when I started having sex I experoenced times when having sex felt great and times when during, but especially after orgasming I felt extremy tight and in pain. I always tell my boyfriend and he understands but I would like to understand if it is normal and I'd love to enjoy a complete encounter once in a while! Have you ever experienced that? Do you have any tips?