I get it -- it's not unusual when you're young to go into relationships and not make any agreements about exclusivity or not, or for people to just assume exclusivity or assume that you're not exclusive. Unfortunately, what happens when you don't talk about it and make any agreements is that soon enough, you find yourself in a situation like this, where at least one person has then gotten involved in some way with someone else, and then....well, it can obviously be a real mess.
Personally, I'm not a fan of being dishonest with people like that: I don't think it tends to make for healthy or good relationships, and you also have to figure that if you are choosing to keep it from someone, that it seems like maybe you're figuring it wasn't okay? If that's the case, then if you want all the relationships involved to have some integrity, then you'll want to deal with them in a way where there's some accountability, you know?
What do you think about perhaps instead putting anything with your girlfriend that you don't think would be okay with your boyfriend on hold for now until he comes back and you can talk to him about this? Then you can talk to him about what's gone down, deal with that, and then, depending on where you two stand from there, if he still is okay being together, work out whatever agreements and boundaries you're both comfortable with from there? It might be handy to look at pieces like this to get some ideas for that kind of conversation:
• A First Polyamory Guide
• Supermodel: Creating & Nurturing Your Own Best Relationship Models
• Hello, Sailor! How to Build, Board and Navigate a Healthy Relationship