Talk to Pediatrician on Mouth Ulcer

my daughter is 3 years old 3 months back she got some mouth ulcer suddenly which spread all in her mouth which get better after month then after 1 month she again having the mouth blisters but thus time not too much intense like before now after every 2 3 weeks she gets her starting from inner lips and tounge we consulted 3 doctors 2 of them said if she is active and eating well then don't worry 1 of them get her blood test done which was normal they also suggest vitamins but still she gets don't know I am very worried plz guide me mane kafi food usk Churwa diye hain mje smjh nhi ara kis food se hora hai external factors she sucks thumb,put everything in her mouth these are the external factors and other she is mostly constipated now its getting better but still constipation some times mucus comes with constipation her voice is also change when this mouth issue starts.