I really hurt my back doing the plow Yoga position (where you lay on your back and put your feet all the way over the front of your body until they touch the floor on the other side of your head). Afterwards there was excruciating pain down the center of my backbone and I am really worried that I injured my spinal cord, because it doesn't feel like it is just the muscles.
Has anyone else ever had something like this happen? I really don't want to go to the doctor if I can avoid it because I have no health insurance and very little money.
I would find a way to go to the doctor. Your back is nothing to monkey around with.
Were you in class when you did the asana? We do several variations on plow in my yoga class, and the instructor is VERY careful to have us do things PRECISELY, with blankets under our shoulders in a specific spot, etc. Inversions are tricky and I'd be hesitant to do them on my own or without a really experienced, qualifed instructor.