i don't know if you should be worried, if they are trying to break you up, that's their problem but... if your bf is that easily taken away from you whether for that woman or other reason, then it's bound to happen and he's not the one for you. That's not fun news or encouraging I know but think of it this way, if he's one to leave you so easily then you should move on anyway and find someone that's actually going to be commited to you . Of course I don't see any evidence that that's what he's going to do, so likely it's a lot of insecurity that is at this point, baseless.

i would be upset at her coming over, like Leed said. Even if he's not your bf, and you know she liked him before, then it was rather bold of her to come by asking for him. Heck a random girl coming to see if he was home is pretty bold... I can understand it making you uncomfortable. If i t happens again I would ask her what she is doing or why she was there. Put her on the spot. That'll make it clear where you stand on that type of action and she may think twice next time before knocking on your door.