<human>: I can't believe I'm putting this on here.. But here goes..

I went to a bonfire last night at my buddy's farm and my ex showed up. It wasn't planned and I had no idea any girls were coming. I honestly believed it was just a normal guy's night where we sit around the fire, drink beer, and shoot the shit. But it wasn't.

We were hanging out and the next thing I know, four girls show up and one of them was my ex. They saw on Facebook that we were having a bonfire and figured they'd stop by and say hi. But they ended up staying the entire time.

After a couple of hours, we were getting low on wood, so I offered to go grab some more wood from the stack and my ex immediately popped up and said she'd help me. So we got into the truck and drove to the other side of the farm, which is where my buddy keeps it stacked. 

On the way over there, my ex asked where my girlfriend was. I told her she was having a girl's night. That's when she put her hand on my upper thigh and started rubbing. Obviously, I got hard and she noticed. As we got out of the truck, she approached me and said she wanted to "help me out." So she got down on her knees and... yeah.

I feel like a piece of shit. My girlfriend has no idea and would never find out. The only people who know are me and my ex-girlfriend. I honestly don't know if I can handle the guilt though.

What's the best way to handle this without ruining my two year relationship?

Can you succinctly summarize the text in 1-3 sentences?
<bot>: Got a BJ from an ex girlfriend at a bonfire. Girlfriend has no idea and I'm debating on whether or not I should tell her.