Back pain when lying down

Coral • Ftm with an unexpected pregnancy that has turned into my greatest joy.
Recently I've gotten back and leg pain if I lie down for too long prior to falling asleep. Is this normal or should I be sleeping in a different positon? I try to only lie on my sides. Currently 21 weeks so my belly weight isnt much. Laying on my back helps but I've been told not to at this point.