Diarrhoea

My 7 month old baby has diarrhoea for close to 3 weeks. Doctor ruled out infection as my baby still can play and smile. Initially we thought he might be teething, but since it has lasted more than 1 week, it might be other issues. We have stopped all solid food as we are testing if solid food might be the cause. But baby is still diarrhoeaing. If anyone has experienced the same or if you think what might be the possible causes, I would appreciate any comments. Thanks so much !