Kegle muscles reacting weird.?

[His girl]

When I was younger my mom told me to do kegle exercises to help control my bladder. When I lost my virginity I never had any problems with those muscles. I have had sex with two other guys and they have both had problems getting inside due to those muscles contracting so severely. I recently got back together with the guy I lost it to. We had sex and he had no problems what so ever. I briefly talked to my mom about it and she said that I just wasn't relaxed enough. I know it has nothing to do with size either. The guy I just got back with is bigger than one of the guys but smaller than the other. I was just curious and wanted to know what could possibly cause this to happen