PLEASE HELP! I threw out my back

I was just at the store and while I was trying to put a 50 lb bag of dog food in the cart, I threw out my back pretty bad. I was in tears in the store. It hurts to walk, bend down, and turn. I can hardly turn or bend down because the pain is so severe.

I have never had this happen before.

I do have a history of back issues. I was in and out of different doctors for 5 years when I was a teenager (I'm 23 now). The only explanation I ever got was possibly a slipped disk, and even then they weren't entirely sure if that was the issue.

I'm wondering at what point I should go to the doctor. Are there things that I can do to help?