Conflicted

I saw on my boyfriends phone he texted a girl a week before we officially started dating to hang out & flirted as well. Prior to us dating we were hanging out & going on dates. We’ve been together for two years but I feel blind sided reading that today. Should I be mad? I know technically speaking it wasn’t official but it kinda hurts to know we were going on dates & I read the dates of these texts as well.