I wasn't exactly doing somersaults before - in fact I've have quite a bad dose of SPD this pregnancy which has kept me in a lot of back pain around the sacro iliac joints and has left me unable to walk any distance.
But I got up this morning with a different kind of back pain - a low ache all over my lower back and lower abdomen - and feeling unable to stand up straight or move at anything close to normal speed. In hobbling around like an old woman 
Yesterday at a growth scan, the baby was transverse position, and had been on 3 previous scans on the last 6 weeks.
Could baby have turned head down during the night? Tis the only explanation I can think of for my new and sudden discomfort / immobility.