The Ex-girlfriend.

My boyfriend and I have no trust issues with one another. Or so I thought. We know each others passwords, we have each other answer texts for us, etc. Well I'm getting a little irritated bcuz when I look on my boyfriends search bar on his facebook his ex is always right at the top. Well I got curious about it today so I checked the history on the search. He has looked her up quite a few times since we've been together. I don't know how to bring it up to him on how much it bothers me. The last time was a few days ago. Am I overreacting and just being hormonal? Should I leave it alone? Or should I ask him about it?

(We have twins due Christmas Day.)