If I had to bet, the staph came from your own body. Like has been said, it is all over your skin. All it takes is a tiny opening and an accidental rub or touch to get it.
I've had it twice, rough stuff. First time it was like a mini-leprosy. I had a quarter size hole in my rear delt a few centimeters deep and it spread all around my shoulder up my neck and on my chest.
The second time was worse, all over my lats and midsection. This time it was under the skin so the doctor had to take a surgical knife, lacerate then drain all of them, really hurt like hell.
Definitely take the antibiotics, but be sure to take the whole bottle. Last thing you want is a resistant strain of staph...