Every now and then part of my pubic/upper thigh area goes numb for a brief moment. what could this be? could it be serious?

Numbness: This is a sign of a nerve being irritated usually from pressure. At your age the most common thing would be restrictive clothing or belts. Could possibly be a low back issue. Recommend follow up with your physician to evaluate further.

Merits evaluation.: It's difficult to know if nerve roots exiting from the spine might be the cause, or whether it might be do to something else. If the symptom continues to recur, see a physician for a careful history and physical exam to search for the underlying cause.