Sounds very 'sore tooth'-like. One of ours suddenly started having issues eating - she would go to feed with normal gusto but then stop and look incredibly uncomfortable for a while, then try and eat again. As it was taking her so long to eat the others were snaffling all the food. I separated her off for a few days and pretty much gave her gruel to eat/slurp until she seemed to have recovered. She was fine after a few days, but if it had carried on I would have got the vet to look at her.