I (F19) am curious about a relationship that my bf (M21) had in the past that seems to have overlapped with ours.

So about 3 years ago, when me and my current boyfriend were together but not together together. I found out that he was messing with some other girl at the same time that he was messing with me, because my brother saw her tag him in something on FB and screenshot it and sent it to me. Now, we weren't in a full blown relationship but we were telling each other that we loved each other and we were having sex and everything. We are still together, but i am so curious to hear his side of the story. When I tried to confront him about it at the time, he basically ignored me for a couple days, until I caved in and moved on without bringing it up again. Do I have a right to know, or am I just asking for trouble by asking him about it? To be clear, Im not trying to make a big deal about it, but I find myself thinking back on that time every now and then.