Hello~I doubt if it is serious, but it is always wise to have issues that are of concerned, checked by your physician, I am glad you are doing this.
I truly think that it could possibly be due to a condition called "Candida" Candida can be caused by taking too many antibiotics as the antibiotics kill the friendly bacteria in our system, thus, causing issues with our gut, this in turn can cause oral thrush or other types of fungal infections. If it is Candida, then you need to get a good, probiotic at a health food store to replace the "friendly" bacteria that has been lost, also, avoid all sugars and yeast. You should start to notice a difference in a week or so.