R.C.
My daughter (14 months) had similar symptoms, after 2 days I had her seen by her pediatrician who told me it was herpes in the mouth and if I waited till day 3 my baby would have been in even more pain. She gave her a prescription for 10 days but told me I could stop giving it as early as 3 days if the symptoms went away. Luckily by the 3rd day on the medicine she was fine and back to her happy self.
My older daughter had cancer sores when she was 18 months, bad enough to even make her lips look swollen. Same pediatrician couldn't give her anything but said some people are just prone to cancer sores and the first episode is usually the worst. Either you get canker sores or you never do. My mom and sister never have had a canker sore whereas my father and I are prone.
Hope she's better by now