If it is a constant pain then this could be a muscle pull due to sleep position and with rest and correct posture it will subside. However if these are cramps and are coming and going then you must consult your doctor.

You can take support of pillows while sitting and sleeping. Weight going into your uterus is putting pressure on your back. Stomach pain (not pelvic pain)could be due to gas or constipation. Take care of your diet. Avoid fried foods and fizzy drinks. You can take isabgol husk powder with water every night to clear your stomach and avoid gas and constipation.

is your sleep position good? http://app.babychakra.com/feedpost/4380 here's a good posture for sleeping