Potentially it's drainage from your sinuses dripping down the back of your throat, aka post-nasal drip. This is a common symptom with both sinusitis and acute sinus infections, and I know lots of people say they have it when they have a cold too. Basically, too much mucus and/or swelling in the sinuses and the body has to put the mucus somewhere.
I have had sinusitis most of my life and it's manageable on this diet, but when I eat super inflammatory foods (sugars, grains, etc.) / don't get enough sleep / skip exercising / have a couple too many drinks, the inflammation kicks up right away and I get horrible post-nasal drip and the ensuing sore throat, and sometimes get very acute sinus infections too. I would guess that your body is trying to fight off the inflammation from your SAD binge, and that's what your sore throat is from. Bone broth and other warm/hot liquids will definitely help, and take plenty of fish oil to help fight the inflammation.
EDIT: The post-nasal drip will also give me a stomach ache and some indigestion too, especially the 2nd-3rd day when it's heaviest. I would guess you've probably got some rumbly tumbly going on with the switch back anyway, but the drainage might also be part of the problem.