I ran into my ex at the gym

I went to the gym yesterday, and I had been feeling great for weeks as I established my routine. But just as I was drenched in sweat, with my hair tied up, I suddenly locked eyes with my ex. I literally froze, and he seemed to have put in quite a bit of effort to look good. Normally, I wouldn't let things like this get to me, but I felt incredibly uncomfortable. What do you think should be done in situations like this? Should I act as if I don't care, or should I say hi?