Ex boyfriend.. in relation to current boyfriend. why does this happen?

My ex has just done the same thing..Hes been talking to me all the time and wanting to hang out. At first I thought it was cool that we were talking and being friends again..but now he wont leave me alone and always turns everything ual..I havent done anything wrong but I still feel bad because I do have a boyfriend and know I shouldnt be talking to my ex as much as I have. But Im a nice person and dont want to hurt my ex's feelings either..