6 Month Old Always Fussing

My 6 month old baby has been nonstop fussing the last two weeks. He had always been a happy, smiley baby and still is when we make him laugh. But most of the time, he has been whining and fussing no matter what: on the play mat, in our arms, in the high chair, etc. Even with a clean diaper and freshly fed and napped. I don't see any teeth coming in yet.

He's been eating well and doesn't show any signs of being sick. He sleeps 11 hours at night so I don't think it's tiredness.

Anyone else experiencing the same??