Hello and thank you for your question.
Based on your description the numbness you are experiencing can be due to neuropathy or can be due to compression of the nerve in your lower back due to your weight.
Neuropathy can occur also due to unknown reasons, vitamin B12 deficiency. I would suggest to see a neurologist who can evaluate you clinically. He may also order EMG nerve conduction test to evaluate whether it is compression or neuropathy.
Please get back to me if you have any further questions.Thank you.