At only 18 months she more than likely isn't ready to be potty trained. Wait until she turns 2 and try again.
Added- Oh, I thought by saying she used to pee in her potty but now she doesn't that you were trying to potty train her.
That is a stage most kids go through, both of mine did about that age too. You just need to be firm and discipline her when she does it and let her know it is not okay.