Sounds like the 12 week wonder week to me. Google wonder weeks and have a read...especially since it's come on suddenly.
There's another one at 12 weeks?? Far out lol. For some reason I wasn't expecting the next one til week 18 but I'll look it up. It would make sense as she was similar with the last two wonder weeks.

If she gets worse I might go to the gp for a check. What would oral thrush look like?