Rash around mouth after drinking formula

DD is 5 mths old. I have decided to start formula feeding her 1 bottle a day and slowly stooping breast feeding.
She has so far had 3 bottles over the last 3 nights and each time after she's finished she gets a red rash around her mouth and chin. It seems to be on the spots she dribbles a bit of milk.
Does anyone have any ideas what could be causing it? It does go away about half an hour after she's finished feeding.

Oh I hadn't thought of it being the teat. Maybe I'm not careful enough with washing.
I don't think it's affecting her in any way although it would be hard to tell right now as she's quite sick with a cold