Is it right for my bf to be friends with his ex?whom i know he loved so much?

i have a bf right now,weve been doing good for almost 4 months now,until one night my friend told me he saw my bf with his ex gf in a bar,but in fairness to him,they come in group because they share common friends,and i asked my bf about this and he said that they just cleared up things and agreed to be just bestfriends..but you know theres a feeling of jealousy inside of me, i dont know what to think or should i get insecured?because i know he loved this girl very much before that he's the one paying this girls school fee,im trusting my bf but not the girl..what should i do?as i can see this girl wants his attention back,i love my bf and i dont wanna lose him..please help me..i just want to make this relationship to last long..thanks for everyone who would help..