I keep getting these bloody mouth blisters and don't know why. I try to get rid of them by scratching with my fingernails but I've got one now that I just can't seem to get rid of. Usually the fingernail scratching works but not with this one.

My Doctor told me to eat Activa Yoghert as it is a mouth infection. I have been eating it but they just keep coming back and now I got this huge one in my mouth, like I said, can't seem to scratch away.

Can you help?