unstable at high speeds.

so Ive been having this problem with my car...at speeds higher than like 55mph, the car feels unstable, like if I take a curve a tthat speed, feels like Im gonna lose control, feels like the rear end is gonna break loose a little just by switching lanes...anyways, my question is what can be causign this? could I have broken an integral part of the suspension system? it started when I took my car to the track, when I was doign 1/4 mile runs, I did a couple, it was fine, then I started noticing that at the end of the track the car was getting unstable..could I have broken something at the track? maybe a sway bar or something?