My car rolled backwards!!

As the title stated, my car rolled backwards on a slope the moment I let go of the brake to step on the accelerator today. In my 9 years of driving, this has never happened. I was completely shocked & stepped on the accelerator with all my might. The car almost "flew" with a loud "vroommm". Scary. Now I seriously need to reconsider driving this car if the place I go to has a slope. I'm not a good confident driver. This car is stressing me out!!