Most Helpful Girl
You two need to sit down and talk. But don't forget to sit down and think about how you feel about this first.
Is this something you can forgive her for? Or does it upset you enough to leave her? The choice is truly yours. From your answer there is no way to know whether this is the first time she has done this, or if there have been issues in the past.
If this is recurring behavior for her, then maybe it's time to move on.
However, if this is the only thing she has done, and you can find it in yourself to forgive her, then maybe it's okay to stay? The decision is up to you :)
You have to decide if you can live with this or not. If it's affecting you, then you need to deal with that. Set up some counselling for yourself. If you truly want to continue the relationship, then it's a good idea to seek out couples counselling.
The only person who can decide this is yourself. The decision won't be easy, you also have to consider your child. But you can't stay together just for your child. If this is making you unhappy and you aren't able to live with it, then your child will be affected by your unhappiness in the relationship.