Why does my ex stare at me after such a long time?

my ex and I broke up 6 years ago,since then I have dated and have found a wonderful guy,during these 6 years my ex has tried to get my attention by staring always being close to me no matter where at the club we are,and recently this Saturday,he stared at me so much my friends were uncomfortable that we had to leave,once we left to another club,which has two levels,i was told that he has been upstairs staring at our group the whole night,meaning "me"and by the way my fiancee was there with us at that club,at the club prior to that it was my friends and I only.another thing I find weird is that his best friend was next to us the whole night looking and kept looking up at the balcony almost reporting back to my ex...so odd.. what does this mean?i know some of you think I should ignore it,but this has been going on for so long that its hard to ignore it anymore.help?