7 months old refusing formula

My baby turned 7 months last week and vefore that everything was fine he was taking 26 to 28 oz per day with 2 times solids.... now the day he turned 7 months i dont know what happened and he doesnt want his formula anymore with a lot of effort and feesing him round the clock i manage to feed him 18 to 20 oz with 2 times solids.... i dont think he is teething and we saw his doc yesterday and he doesnt have any infection or cold etc as well.... have you experienced something similar and what worked then?