Am i worrying about nothing?

Recently I've noticed that my boyfriend has become and is best friends on snapchat with his friends girlfriend. I've been with my boyfriend for three years and he did not know this girl until recently (4 months) when she started to work where he works. I cannot understand why my boyfriend and this girl would have a reason to snapchat one another... Do I sound like I am overreacting? We set rules out at the start of the relationship where we would only have our close friends on snapchat as it's quite a private form of social media where you send images to one another. I know for a fact that if I added her boyfriend on snapchat, she would not like it and my boyfriend would not understand the need for it. I feel like I should be more relaxed about it, but then my gut instinct isn't great and like I've already said I really don't know what they'd both need to send one another.