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

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.
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.