I am having canker sore in teeth root and also tonsilitis ,i took amoxcyllin for 3 days after that suddenly getting high fever what could be the cause?

Possible infection: Your mouth ulcers are viral and are usually associated with lower grade fever. The fact that you are having high grade fevers is concerning. You may have a secondary bacterial infection or abscess . Such infections can be life threatening. See your physician immediately.

Stop amoxicillin: As far as I can tell, a strep infection has not been documented thus the question as to whether you need Amoxicillin. I suspect that you are developing a drug fever to amoxicillin and need to discontinue it immediately before you get sicker.