I saw my ex today.

So I’m 16 I had an ex bf over a year ago that I lost my v card to. I now have a bf of 9 months and he’s amazing. But ever since I saw my ex... I like can’t stop thinking about him?? WHY?? Like I was just shopping and we just saw each other and made eye contact and didn’t talk but I got like those weird nerves and a butterfly feeling and idk why! Like I love my boyfriend and he’s amazing. But ever since I saw my ex this brought like flashbacks and I can’t stop thinking about him now and ugh🤦🏼‍♀️ please help a girl out. Idk why this is happening.