Slip of the ex!!

My boyfriend and I have only been together about 5 months but we are both in our mid 30s so we know we are very serious about the long term of this relationship. 
​So we have been arguing over some petty issues and he slips by calling me his ex's name. She broke it off with him about 7 years ago and he hasn't had a really serious relationship since. 
​I feel so mad and cut that he could call me her name. 
​Should I be super upset about this? Or am I over reacting??