Recurring Thrush, no reason why

I've gotten thrush for the third time. First time was March 2015, I was attributing that with using Thieves essential oils in my mouth on what I thought was a canker sore. December 2016 got it again, figured it was from the homemade chapstick I received from a friend and used, it had essential oils in it. Got it again April 9th, but have not been near essential oils. Nystatin mouth rinse I've used and a pill, but I don't have my paperwork here to say what they were. Completed my meds each time in full. Did blood work this time, all are normal. No HIV. Dentist said my mouth is very healthy, gums and such. All are puzzled. Any thoughts on this? Thanks! Rhoda