Well shit. I made it through the entire pandemic up to this point. I've had two vaccines and a booster, I still wear a mask in crowds/stores (not that I go into crowds aside from stores to get groceries) and I just got back a test my doc made me take because I had a sinus infection (I thought) and here I am... positive for COVID-19. Delightful. Irate now.
the masks were never to protect us, they were to protect others from us, so more than likely you just came in contact with someone who wasn't being cautious
honestly you did the best you could, and now you know you are positive so you will stop the infection chain. You sound like you were being super cautious and you will probably never know how you caught it.