You seem to be experiencing an abdominal muscle spasm
as a result of stretching and exercise. The weight of the growing uterus
leads to stress
on the back muscles also leading to pain. You should be able to feel if the pain is coming and going or staying all the time. Also, associated vaginal discharge
or leaking, should also be observed. Stretching of the ligaments is the most likely cause for you. You can try local analgesics for the back pain
and while getting up from the bed try to get up from a side. Practise proper posture and exercises after consulting your obstetrician. Take care.