Is it normal for me to care about this?

I'm getting married next month and I really love my fiancee so much but lately my ex girlfriend has been trying to mess with my head and its working. I dated my ex from when I was 14-18 (Im now 21), and when we were 17 we had a baby together. I started dating my fiancee when I was 19 and Ive been so happy ever since and never even thought about being with my ex again, but my ex has also been obsessing over me since I broke up with her and has said to me numerous times "Im never going to move on, I'll wait for you to take me back" and for the past 2 years she hasnt even talked to any other guys as more then friends. Now all of a sudden last week she started seeing this guy who I don't like, their not in a relationship yet but there close to it and I don't like this at all. Its not like I still have feelings for my ex because I dont at all but thinking about that guy in her house around my son really pisses me off, I cant even think about it without wanting to go fight the loser. Is this normal for me to be thinking this way? I feel like she is trying to mess with my head just to confuse me right before my wedding, because she is deffinately the type who would do that just out of spite. As I said before this dude started hanging around with her I couldnt of cared less about her and now its really bothering me. Is it normal for me to care if she starts a new relationship even though I dont even like her?