4 month old with watery stools for 2 days?

I had to switch his formula a couple of days ago because my WIC office became retarded and changed his formula on the vouchers, so i had to get what was on the voucher at the store (i shouldn't have anyway). He's been on Similac Sensitive since he's been born, but then was given him Similac Advance 2 days ago. Ever since he's been switched he's been having real bad watery stools like every 2 or 3 hours in the day. I rushed to the store earlier to get his Similac Sensitive thinking he might be just having a bad experience with the other one...or maybe Lactose and tolerant. I have 2 older kids that never had this experience. But should i give it a couple of days for the Sensitive formula to just kick in and may help? Or is this a sign of Diarrhea? I was told to pinch above his belly on to make sure he's not dehydrating. He's just not his happy self anymore, and he cries when he poops and has ALOT of gas with it too. Has anybody had this experience with change of formula?