An MRI will not miss a hernmiated disc, as zlundbe stated.
The left buttock and leg pain are classic pains characteristic of inflammation of the sciatic nerve.
This pain does not respond to opiates. They are inappropriate. It responds to anti-inflammatories and a program of special exercises.
A TENS device is also helpful.
Do a search on "sciatica" on Medhelp and on google.
First, an MRI should not miss a herniated disc (but there are bad doctors, like all professions). If the numbness/pain has been continuous since your last epidural it could be peripheral nerve damage from the anesthetics, but not likely. Your probably just experiencing muscle tension in your buttocks/thigh creating nerve compression, most likely piriformis syndrome. I would look for some form of muscle therapy (PT, chiropractic, massage) whatever your most comfortable with.
Good luck