advice please!!!

Okay so today I went to the gym with my boyfriend so he could play basketball, and my ex of 4/5 years ago was there! It was never anything serious, we never even hung out or any of that type of stuff. Well when I seen him today I noticed that he kept looking at me, sometimes direct eye contact and sometimes I would even catch him looking at me when I wasn't paying attention. I feel so bad because I can't stop thinking about it, because idk i guess it made me feel like he thought I had gotten prettier, because I have changed quite a bit. maybe he just thought I was staring at him.. idk but I can't get it off of my brain! And now I kee thinking about him and I feel like that's wrong. Advice! Ps sorry I keep rambling😂