Not sure what to do.

So I've been with my SO for a while. We have a pretty great relationship. Except, I've been keeping a huge secret from him. I decided to meet up with some friends & my ex BF was there. While there we were just having a lot of fun & had a drink or two. I wasn't feeling well so I laid down in my friends bedroom for a while. While laying down my ex walked in the room claiming to just check on me. I told him I was fine & to go away but he sat down next to me & rubbed my head & started to talk to me. I tried to leave but he grabbed my arm & forced me on the bed. He tried kissing me I told him stop, he took off my pants & I yelled at him. Trying to struggle to get away & kicking. He forced me to have sex with him, he legit held me on the bed & forced me. I have not told my SO. Should I tell my SO I feel like he will be mad that I was even in an area where he was & didn't just leave. I feel like he won't understand.