I think my husband is cheating but I'm not sure. how would you proceed.

Today I was on my husbands instagram, my phone has been broken and he lets me go on his. I accidentally swiped and ended up in the messages. I was going to swipe out but I saw a message to this girl  and he said to her " hey what's your Snapchat angel?" So I don't have access to his Snapchat on his phone  but intended to look on his tablet later now. 
I'm really hurt. I saw he sent this yesterday. On Father's Day. Right after we had just bathed our 5 month son together and put him to bed. Am I overreacting though ? Should I bring it up ? Should I look at his Snapchat first ? Is it cheating what he's doing ? I looked at the girls profile and she posts pictures of herself sexually and doesn't look like they know each other.  My husband and I have our problems but yesterday was such a good day I thought and his first Father's Day. I'm just confused ....