My 3 month old is never happy!

This is my third child, but by far my most unhappy :( he is 3.5 months old, exclusively breastfed. I don't know what's wrong with him. He just starts screaming out of nowhere and nothing I do seems to help got more than a few minutes. I've tried burping him, gas drops, switching positions, singing to him, swaddling him, etc and he'll stop for a bit but then he starts up again. His tummy isn't hard, he doesn't have a fever, doesn't appear sick. He wasn't always like this...he just started up with this about two weeks ago. Any ideas?