My six month old has been extra fussy, sleeping less, eating less and has diarrhea. no fever, what could be the problem?

Teething for one: Teething is certainly a possibility at this age, and can have similar symptoms. But other things are possible, including ear infection. If it continues go see your pediatrician.

Several possibles: A minor little stomach bug - 'stomach flu' could cause some irritability decrease in appetite and diarrhea. So could other minor illnesses that go around this time of year like hand/foot/mouth disease and roseola, although roseola usually has associated fever. Also could be from something like teething. Give lots of tlc, push fluids, and monitor for any new symptoms.