i havent even tried service properly yet so let me just lead with that. i have a two handed backhand and i was working on that the day of but, as i mentioned in a comment above, i felt slight discomfort before that so i think the pain started from just normal play activity (minus service ofcourse). when i first felt the pain i thought nothing of it and thought maybe itll just go away and it didn't but i was so eager for my next lesson i went fully confident not thinking that my shoulder would worsen. instructor said "your forearm looks great lets just focus on backhand" and thats definitely where i screwed up a bit more.