Should I blow up on him or keep calm?

I just found out that the guy I've been seeing for 3 yrs has also been seeing another girl for approximately the same time maybe even more. He just got back from out of town from being with her and has no clue that I know. I haven't reacted the way I expected, no tears, no scenes no drama. I'm wondering if I should blow up on him telling him what I found n brake it off? Is it normal to not want to tell him anything and remove him without explanation? I'm just surprised I have no emotions for this