Pelvic pain when laying on back

22 weeks and for weeks now mainly in second trimester having pain in pelvic area when laying on my back: it’s not severe but it feels like pain inside that strikes every couple minutes. When I turn to side it goes away. Anyone have that and knows why it happens or if this is normal? It happens even if I’m propped up on a two pillows with my head and small part of upper back elevated.