I have been feeling very distant from my girlfriend for the past few days. I need help.?

Ever since we had for the first time on monday I have felt different. Monday I felt great and extremely close to her but then things...changed. She says she feels closer to me than ever. I'll start to feel better for a couple of hours each day and be truly happy with our relationship but then I just spontaneously feel bad again and feel as though im losing touch with her and losing our connection. I really love her and want to stay with her. Im just not sure what is wrong. Please help.