Weak Teeth . . . . . . . . . . . . . .?!

Right I used to have really strong teeth, and then I had braces to straighten them out. I had my braces off about 1-2 years ago but ever since then my teeth have felt weak. Lately they have been feeling really weak and move slightly. I clean my teeth everyday, twice a day, and I use floss and mouthwash so i really take care of my teeth. What could be making my teeth really weak? Is there anything I can do to make them stronger?

Thanks!