Help!!

Okay in my 99 Avenger, when you get to much weight in the car or even pretty much one person in the back ( I had my friend in the passenger seat and my little sister, whos like 10, in the back seat) it sounds like, when you hit a bump, it is bottoming out or something? It either sounds like the back end of the car is bottoming out or the wheels are coming up and hitting the wheel wells?