It sounds like it might just be a part of your skin- if it's something that bothers you cosmetically, you can pursue seeing a dermatologist (another one if the doctor you saw was one) to see if they can do anything about them. The signs you want to watch for, for health reasons is itching, pain, or sudden growth or changing of the spots.
Hi, I've been dealing with EXACTLY what you described! Haven't did any info online... Started about a year ago. Sandy feeling only goes away if I use listerine or salt water gargle. But will then come right back if go a day without that. I'm worried it's a fungal infection and don't have insurance. Did you ever get yours cleared up and find out what it was?