I gained 12lbs with my first daughter, I was young as well. I had alot of back pain throughout my whole pregnancy only thing that helped me was a hot shower or bath. Not much you can really do. Sometimes I'd get a tennis ball and have my mom rub it on my back, back and forth.. It helped a Lil.
I have the same problem I am on baby #2. N my first was severe I could not walk much. But its normal