Child with canker sores

My daughter is 9. She keep getting cancer sores and I'm not sure why. She seriously gets them at least 2 times a month at times. She woke with morning saying her mouth felt swollen, I looked inside and sure enough 3 little canker sores in one spot. What causes these? Why would she get them so often. Makes no sense. She just recently had blood drown and everything was normal so it's not a deficiency. It's not from her biting the inside of her mouth because the area she seems to always get them is areas she can't bite, it's not her cheeks she gets them. As someone mentioned about citrus, it's not from that either as she has acid reflux and can't really have citrus stuff.