I think she is cheating on me.

His Sis Dolly, I am a 36 years old man and my girl friend is 29 years we have two kids.When I start having sex with her she is wet and I can penetrate her without difficulties and as we are having sex she runs dry in a way that I penetrate her with difficulties and I could feel my penis hurting. I do’nt understand because I believe that at that stage it should be the time that she excrete virginal fluids. So to me I think that she is cheating on me, because if she can’t release virginal fluids then she has released them before and thats why she is getting dry.