Numbness in hip area when riding

When I am riding my bike (on the road or on the trainer) the area just above my right hip in the lower back gets numb after about 30 minutes of riding. It doesn't go any farther than that, just that small area. I have no other pain, numbness, etc., in any other area of my body.

Only been training seriously for the last month and I was fitted for the bike.

Any ideas what might be causing this?

Thanks in advance for any responses.