In your situation, Id have a lot of questions and thoughts.
(firstly, Im really sorry to hear that, thats terrible).
When people suddenly change overnight, it usually isnt anything to do with their boyfriend/girlfriend. YOU hadnt changed, so something in his head did..and something must have triggered that...has anything else happened in his life lately that is major and may have made him re-evaluate (maybe rashly) everything?
an example....I broke up with someone once because I met a guy and his girlfriend who were clearly so right for each other and i thought *me and (exs name) are never going to be like that, oh g-d, must end it NOW* so I did. And I regretted it.