Got any hip problems? Might be pinching on a nerve.
Hello and welcome to the forum. Thank you for your question. The most likely explanation is nerve impingement. Since this only happens in a particular sleeping position, this could well be the case. That changing positions alleviates it is a good sign. However, other things also can cause this would have to be considered if this continues and it is not the simple explanation of a compressed nerve. There is a specific kind of nerve impingement that can cause numbness of the outer thigh called Meralgia paresthetica. Are you overweight by any chance? It is more likely if this is the case and losing weight in itself can greatly help. Unfortunately, nerves can not just be compressed but actually damaged. Femoral nerve dysfunction is an example of this. However, that it goes away when you change positions makes this unlikely. Talk to your doctor but would guess this has to do with sleeping position and nerve impingement. https://www.healthline.com/health/numbness-in-thigh#treatment