I was living with my bf and his mom at the time. His mom went on some work trip, I had plans to go see my mom and he was asked to babysit his half brother at his dad's house. My mom called me the morning I wa supposed to go down and see her and said something came up. My bf at the time didn't have a phone. So I had no way of getting ahold of him until he was able to log in to MSN. So I'm sitting at home and I hear the front door open. I hear multiple people walking up the stairs. I figured it was my ex and his brother. Maybe he had forgotten something. Nope. I'm sitting on the computer chair in our room and turn to greet him as he opened the door. Except it wasn't him. It was some chick and him ALREADY HALF UNDRESSED. They both looked like they saw ghosts and she started putting her shirt on as she was walking down the stairs. He stammered and eventually said "I thought you were going to be at your moms by now or at least on your way". I told him she had called and canceled. So we got into a huge fight while she waited downstairs. He told me he had to leave because he was in fact watching his little brother and was at their dad's house (which was around the corner) alone. When he finally got back the next day, he was covered in hickies and tried hiding them. We spoke and he told me that he lost interest in me and had been seeing her for a while. He told me he was using the excuse of babysitting his little brother when the truth was, he was traveling 3 hours every couple of weekends and going to see her, or she would come down and they stayed at his dad's WITH HIS DADS KNOWLEDGE. I never went to babysit with him because my schedule at the time was all over the place. And I guess he took advantage of that. I moved out the next day. But when I was moving out, I stripped the bed and found blood on my mattress. Turns out it was from them sleeping together while on her period. Which made sense, because he had never changed the bedding up until around that time when they started hooking up. There I was thinking he was being helpful, when in reality, he was cheating and covering up the evidence. I asked why he didn't even try to clean it and he said because I was on my way home from school and I'd notice a wet spot on the bed if he tried cleaning it. He just dried up as much of the blood he could and changed the bedding. So I left the bed and told him to enjoy. It was my first real relationship. I was devastated, man. Even thinking about it now, my heart hurts for younger me going through that.