What Causes Recurring Strep Throat?

Hey, I've had strep throat since Friday, the whole time I've had a low fever of 99. I went to see the Doctor Monday and I was givin Amoxicillin to take twice a day for ten days. The strep throat has gotten worse, I have more yellow/white dots on my tonsils and they're more swollen. This is also the third time I've gotten it this year, and it seems to get worse everytime I get it. What's going on???