Back pain from shoveling?

I'm just a teenager by the way, so yesterday I went to my grandparents house to shovel their driveway. It had not been shoveled at all and we recently had a huge snow storm leaving us up to 9 inches of snow and a lot of heavy ice. I was out for about 45 minutes and went inside to rest up for about 30 minutes. It was extremely cold outside and was also flurrying (probably only about 10 degrees F with winchill making it about 3 F) My back didn't really hurt the first time but I went back out for about an hour. The snow was extrememly heavy and I was getting a ton in the shovel so I literally had to carry the whole shovel to dump the snow. Then I had to go to the front of the driveway with huge chunks of ice and it hurt when I bent down. The snow was so heavy and hard in some places I had to kick the shovel in the ground so it would pick up snow/ice. I could feel it putting pain on my back and I think I pulled a muscle in my tricep area as well. This was all yesterday afternoon and last night my back was getting pain (mostly the lower to mid back area) it hurts when I bend down and move for the most part, I put on a heat thing last night and it didn't help a lot and this morning I woke up with stomach cramps, throbbing headaches that kept leaving and coming back and the back pain too. I decided to stay home (which I am at home now) and I'm deciding to go back to school around lunch time. I took some eibeprofen earlier too. But something I'm scared of is my backpack is 20lbs (literally) I don't use my locker and it's been like that for 2 years. My ribs sort of hurt when I stretch out and I'm afraid my backpack will really hurt even more since I have new back pain. I will start using my locker if needed but can someone tell me whats going on? did this pain happen from shoveling? how long will it last? how can I make it go away? thanks