Back Pain

I've been experiencing an aching pain down the right side of my back for a few weeks now. Some days it's worse than others. I was at a very healthy weight before pregnancy and have only gained 8 pounds so far. I am 21 weeks along and my doctor said my weight is right on target, so I don't think it's due to excessive weight gain. But holy moly the pain is horrible at times. I've tried a heating pad and it hasn't helped much. If this is what it's like at 21 weeks, I'm fearful of what the remainder of my pregnancy will be like! Has anyone else experienced this? If so what helped you?