2000 Buick Regal My car shakes

My car began shaking a tiny bit while driving a month or two ago. I had the tire pressure checked and that didn't do anything. Now it shakes much worse. It is a gentle rocking when going slowly, goes to a very serious shaking in the middle speeds, and a hard vibration at high speeds. I am wondering what it could be so that I can take it to the right place. Thank you!