I have a white patch on my tongue but swab for oral thrush came back negative. what could this be?!

See a dentist: It is hard to tell without a clinical exam. Oral lichen planus, oral thrush, leukoplakia are some of the possible conditions. You should see a dentist asap.

Many Possibilities: Don't freak out yet. The fact that there does not appear to be a causative organism is part of the process. Ask an oral surgeon to have a look. They will be able to judge if a small sample needs to be taken. It is not the big "c" yet! it might just be biting. Best wishes.