I Have Been On Optifast For 9 Days And On

I have been on optifast for 9 days and on day 6, I experienced what felt like bruising on my hard pallet. Later that day, I developed swelling on either side of my hard pallet close to the soft pallet. When I feel it with my tongue, it feels like a raised line on either side. I haven't been sick in over a year and I haven't had food aside from the shakes. I am having gastric bypass in 2 weeks. My dentist took xrays and confirmed its not dental and put me on an anti-biotic. My nurse said I should try Benadryl and if it works I should stop taking shakes and work out a low carb diet with my dietician. Both my nurse and dentist are completely stumped and I haven't found anything like it on Google. Any idea what it could be?