as described by you, sore throat
along with small white patches on tonsils depict tonsilitis
.your strep test
was negative which cant confirm its not tonsillitis
because tonsillitis can be either viral or bacterial and in bacterial it dont only involve strep as its causative agent. so continue with keflex. also do warm saline rinses and take warm liquid stuff as much as possible. it will take atleast a week to properly heal. so dont skip your medicine.
wishing you speedy recovery
with best regards