From what you have described, this could be thrush. This is an over growth of a fungus that is normally found in the mouth, called Candida albicans. It generally looks like cheese curds, which when wiped off, leave a raw, bleeding surface. Please check with your doctor to know for sure if this is thrush and to be started on prescription treatment. This is treated with Nystatin Oral Suspension or Mycelex® (clotrimazole) Troche.
This could just be a viral infection. If so, this will spontaneously resolve. It could still be strep, despite the negative cultures. Since your symptoms are continuing you might ask your doctor about getting another throat culture for strep.