What Causes Numbness In My Right Feet?

I had a prolapsed disc about a year and a half ago and I was told at the time by my consultant that it was minor after an MRI scan (I did have a bit of numbness in my rt.feet at the time), and that I must't do any taxing physical activity or lift anything heavy for about 6 months. I sometimes still have a bit of lower back pain when I lift something heavy, and am just wondering if it is safe to lift weights at all eg 7-8 kgs at times when travelling?