He changed so fast I don't even know?

So my boyfriend and I had been going out for 3 weeks. He was so into me we texted 24/7 allll the time, he always used lots of exclamation point an smilie faces. He always complimented me (I wasn't like promptinghim or anything), and he always told me how much he reallly liked me and how much he cared about me. Then overnight he changed I don't know how but then he stopped using exclamation points then smilies then stopped saying lol,then stopped replying sometimes, then it led to breaking up. He is acting so different we haven't talked since, but even his friends said he's been acing jerky, so they noticed his change. What do you think happeed?