I need help with this... ❤️

Abi
Hi! Im Abby, and I have a very long story to tell, which I need help in understanding. Please read the full text and hopefully I can come to terms with why this happened to me...
So my story begins about a month ago. My boyfriend at the time and I were happily together, and we're getting ready for going to the Halloween dance together. On the Wednesday before the dance, I was score keeping a volleyball tournament he was in, when a guy on another team made a comment about how my "tits bounced when I walked". I didn't hear this comment made, but he did, and he was mad. He started to yell at the guy, telling him to keep his eyes on his own prize. When he came to talk to me about it, he was furious. I didn't know what happened and he couldn't explain why he was so upset because he had to go play. He proceeded to struggle in the next game, and they lost brutally. Afterwards, I asked him what was wrong and he was so upset he started to swear very loudly, which the ref said he needed to calm down. He freaked out at the ref in anger, and explained what happened. The guy who made the comment apologized to me and suspended for the game. Afterwards he played great, and they won the next game. 
Later that night, we were getting ready for the dance at our own houses, and I asked him to FaceTime while we got ready, which he replied "yes please ☺️". We called, and when we were all ready, we were on our way! Now we were only going to be at the dance for about 40 minutes, as he had a hockey game to go to later, and I was going with him. Our night was great, we won his game, and we were great. We then went back to his house for a bit,and I had a little breakdown. My mother had just been diagnosed with cancer about a month before, and I just had a mini freak out. We then were all good, went out to a party, which was great. Neither of us drank, and we went home with the biggest smiles. 
The next day, we were both busy for most of the day, and we hung out later that night. We were all great and happy again, but he then found on the phone some messages (snapchat, and about 3 sent and 3 received) to a friend of mine I made when I was on an exchange a few years ago. He was never really comfortable with the guy, and we had a lot of girls/guys talking to us issues in the past month or so, but nothing that broke us. He got really upset, and I felt awful inforget to tell him we had spoke a little bit. Now this guy I never talked to because I knew he wasn't comfortable with it, but all we did was sent pictures of instruments back and fourth. That was literally all that was said. 
I understand this is a LOT to read, but I promise I will get to my question soon ❤️. 
He was very upset, but forgave me, and became very romantic, which he hadn't in the past month or so, and convinced me to have sex. It was very romantic and I finally began to think we were back to our regular happiness ☺️ but he became upset over the boy messaging me afterwards, and I went home. On my doorstep he told me he forgave me, and exclaimed how mucuh he loved me, and how he wanted to be with me forever. I went to bed, and the next day we barley spoke. Later that night, he broke up with me. He told me he loved me and he always will, and said some weird stuff about if he dies if I'd come to his funeral (he wants to go to the military). He has still yet to give me a legitimate reason as to why, and we are going to exchange our stuff on Sunday (in 3 days). 
I am still heartbroken. I never saw it coming, and never thought it would happen. We were together for over a year. We have sort of spoke a bit, he and I performed a song for an assembly with the band, I sang and he had a cello solo. Which when I got dressed up and stood beside him, he couldn't play properly. He messed up badly on the solo, then I spoke with him after a bit, and I asked if we could talk about our plans to exchange stuff, and he asked me out to lunch. I said yes but I was postponed, and he then told me how I did great and I looked beautiful. I said thank you and that was it. This was about 2 weeks ago. He then proceeds to look at me a lot, he gets nervous around me, and I don't know what to do. He tells people he is done, but I didn't think the man I loved would be capable of giving up on something I thought he loved. 
So what this comes down to is, why is he doing this? Why did he do this? Can someone just fall out of love like that!? Can someone just 'loose interest' in a day. There were no warning signs, no clues as to this would happen. 
What should I say to him on Sunday? What should I do. I know I have to move on, but I have this gut feeling he is regretting it, but trying to convince himself he did the right thing. 
Thank you for reading, and please I ask for help, advice, or even share your stories! 
Thank you!! 
Abby