I had an intermittent numb spot on the front outer part of my thigh for a while. In my case it related to laxity of my pelvic & sacroiliac joints (ie like SPD) & my hip was out of correct alignment too. This must have been pressing on one of the nerves that travel through the lower back. Seeing a physio & doing exercises to strengthen my pelvic, back & tummy muscles fixed it for me. A friend had similar symptoms & hers was bursitis so there are probably a few possible causes.