There are OTC meds for canker sores, but as she is so young I would ask the doctor to take a look, has she got any teeth coming through at this time , could be causing some pressure on her gums . Make sure she is getting liquids as its the hot weather , and you dont want her getting dehydrated maybe some popsickles will help soothe her mouth ..Good luck
I get these pretty bad also. One thing that helps is lysine. It is an amino acid that you can buy most places. You can give it to her in just a little bit of liquid. It really cures them quickly.
Had this trouble when my son was 2 1/2 also. Doctor told me to let him swish and spit Mylanta or Maalox in the bathroom sink. He actually didn't even want to eat because the sores hurt. My boy thought it was fun to spit in the sink. Good luck!