I've been on bactrim (sulfamethoxazole and trimethoprim) for my sinus infection for a couple days. Yesterday after dinner, I spiked a fever out of no where. Is this a side effect?

Infection-related? Fevers are usually related to the medical condition but, yes, can be related to medications, as well. See the provider who prescribed the antibiotics to help suss out whether you're getting worse or having a reaction to the medication.