Need help to understand my feelings. Me?

Hi. I am in a relationship with a girl since last 2 years and we have decided to get married. Mine is a healthy relationship and I am very sincere about my feelings for her. We had a strong bond of trust between us. We are also having a healthy sexual relationship. Of late, there were 1-2   times I felt sexually aroused seeing pics of other women dressed up hot. I have no intention of cheating on my gf neither have I fantasized about having sex with another woman. I was not comfortable with my body's reaction. Even though it was a momentary feeling(sexual arousal soon got overtaken by guilt).I opened up this to my gf. Now,she doesn't trust me, she feels like I will cheat on her. Is it a common for a man to have such feelings? if yes, i csn neglect those feelings without any guilt.Or is it any problem with me?  I truly love my gf and can't think about having another woman in my life. please help me, i cant lose my gf because of these feelings i go through.