My boyfriend likes his ex girlfriend's selfies on instagram?

[Democritus]

You're not overreacting. It would raise anyone's suspicions. He sounds a bit needy, like he's still trying to get his ex's approval after she cheated on him. Tell him she's his ex for a reason, and he needs to move on.

There are only a few valid reasons to stay in touch with an ex:
1. They have kids together
2. They work together and cannot avoid each other.

If he doesn't have one of the above excuses, then it's a clear sign that he has not moved on after their breakup. This is a bad sign for you. I would worry that he's just using you to try to make her jealous. I would make your boundaries clear, and find out what's really going on here.