Ive had a dull ache approx below my cheekbone, around molars, right side only for 2 weeks. hits in afternoon. im guessing teeth grinding most likely? mouth guard from drug store good place to start?

Reasonable yes: If the pain is not severe, no fever, not interfering with eating, then trying a mouth guard from drug store is definitely reasonable. Otherwise may need to try to see a dentist although it is difficult to get in right now with COVID 19

Toothache: Most common cause of toothache is infection. Second most common is fractured tooth. Rather than choosing a "1 size fits all" remedy that may actually cause damage to your bite, do it the right way. See a local Dentist. Have the cause of your symptoms diagnosed. Then treat the cause, which will resolve the symptoms.

Good Guess...: But you'll need to see a dentist to know for sure. Store bought bruxism appliances don't really do the trick. Could be periodontal issues, fractured tooth, loose filling, decay, etc etc.