why?

Is it a sin to be jealous? Is it wrong to be jealous if u found out that ur ex is staying in a same house with a girl who he claims to be his "friend"? When a girl and a guy living together in the same house... anything could happen especially when she is a hottie! I couldnt explain how i feel right now. Does that mean i still have feelings for him and maybe that's why I'm jealous even though i have someone special to me right now? gggrrrr... please help me!