Why do i keep getting canker sores in my mouth?

okay so for the past month i have been getting these random canker sores on the insides of the cheeks of my mouth. my last dentisit ap. was last year and they said i was okay expect i maybe need to brush my gums more. i guess i have sometimes by not really. i realized that when i lift my top lip up, the gums seem alittle darker. but they are fine to me..maybe im just over thinking or something...but really what would cause this? they hurt a lot and sting everytime i touch them with my tounge (hurts expecally when i eat) i have got this cream that numbs them and it works..but still this is inoying! p.s. gonna go to the dentist soon but gotta wait till my mouth clears up