Discectomy L4 L5 three months ago. I had almost no leg pain after surgery. Last week severe pain came back down my leg could this be nerve damage?

Nerve pain. You need to see your spine surgeon again. It is most likely nerve irritation, that could be treated with an epidural injection and conservative care. But, it could be a re-herniation of your disc. Important to rule-out new neurological changes. Your surgeon might decide to get a new MRI after they perform a physical exam.
Muscle Spasm. More likely than not you are having muscle spasms that in fact do irritate the nerves which travel through the affected muscle. Treating the spasm with ice, anti-spasm medicine like tizanidine, pt modalities like electric muscle stiimulation and ultrasound as well as trigger point injections can provide dramatic relief. Defer epidural injections as a last resort. Talk with your surgeon or pain doc.
Recurrent herniation. One of the primary possibilities is a recurrent disc herniation, which occurs in about 10 - 15% of patients. Alternatively, there may be scar tissue developing at the surgical site, or you may have a disc herniation at a different level. See your physician if symptoms persist.
Nerve inflammation. It is possible to have pain going down the leg, after surgery, for a number of reasons. They can range from re- herniation, to a little disc inflammation, to muscle spasm pressing on the nerve. Often this can resolve with a short course of anti-inflammatories and an individualized core strengthening program to prevent future recurrences. Go back to your original doctor for re- eval.