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Pelvic tremors can be due to PID or pelvic inflammatory disease (in females), compression of spinal nerves or due to pudendal neuralgia as an initiating symptom. Menopause and hormonal changes in females too can cause pelvic tremors. Please consult your gynecologist to start with. You may need to consult a neurologist too.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
It sounds like strong muscle spasms. Check with your family physician or ob/gyn to get checked out. Good luck!