Backache problems

Every morning I wake up with a backache and it seems like no matter how I lay (on my back/ on my sides) it's wont go away the only thing that helps is to get up and sit up straight it even happens if I lay down and take a 3 hour nap. I'm only 21 weeks is this normal? My belly is not very big yet so I don't know what the problem is can anyone suggest why this may be happening and give me so tips on how to ease the pain