What Causes Swollen Tonsils?

Hi, I've had swollen tonsils for about a month now. I went to the doctors a while ago and they gave me penisillin for strep throat but the strp test came back negative. The swelling has gone down and the white spots that were there previously have gone away but I'm worried that the swelling isn't gone yet. I didn't have a fever and I am concerned that I could have gotten an std in my throat and that's why the medication didn't fully work