"but he's best friends with my ex boyfriend, who fell in love with me. should i be worried?"

This all depends on your feeling towards your ex boyfriend, how recently you two broke up, and if you really truly are only with this guy because you like him and not to make your ex boyfriend jealous.

Put yourself in his position to see how he would feel. But if you really do like this guy (your ex's friend) then go for it. Try your best not to rub it in your ex's face though. It's going to be hard for him no matter what, especially if this is the first he's hearing about it.

The only situation where you should absolutely not date him is if you're only with his best friend to make your ex jealous. It's a natural thing for girls to do. Dating or hanging out with their ex's friends to cause jealous, and for some weird reason it makes us feel better about things. But like I said before, put yourself in your ex's position and consider his feelings.

But if you really do like this guy and are doing it for the right reasons, then go for it. Your ex may get hurt, but everyone gets hurt at some point and he'll be alright. Just make sure you do it in a nice way.