When i sit one of my legs tends to go numb. sometimes it's the left, other times it's the right. never both at the same time. what would be a potential cause of this and why would they alternate?

Where does it go numb anteriorly or posteriorly? Numbness usually reflects nerve compression. May be a form of sciatica from lumbar nerve compression ( check MRI lumbar spine) may be piriformis syndrome , or if you are heavy /have a pannus(apron of skin) may be from compression of the lateral femoral cutaneous nerve -meralgia paresthetica, or from nerve compression in the pelvis. F/U neurology