My best friend is keeping something from me?

Okay my ex bf and I got back together as friends now we are like best friends. Anyway, i was texting him yesterday and all he was saying was yeah idk. He never does this! He always tells me everything! So i asked are you mad about something he said no, just kinda sad . And I sad o... why? he was like I can't tell you. I asked him why? you tell me everything. He said it would be to hard. And then he told me he had to go. WHATS WONG HERE?