Need Some Advice please??

The first time when i had s** it really didn hurt too bad and then i had s** with the same person about a month later and it didn hurt at all. Then i had s** with a guy who is now my ex boyfriend and it was fine no pain at all, but then i did it with a guy i use to talk to a while but we never had s** and when we did it hurt and took a while for him to get it in. Now im with someone else and we tried to have s** but it was hurting and he couldn get it in either and im just wondering why now all of the is it hurting and hard to even get it in?