It really does sound like a pinched nerve coming out of your lower spin. I'm not a doc but its what I have had for the last 30 years so I've watched the progress. I'm also diabetic so always think about neuropathy.... thought I had it when my shins started to numb but its from the spin/nerve problem. I also get the alternating weak legs. There are all kinds of symptoms from the same thing from pain to numbness to weakness and it moves around. It depends on how hard the nerve is being compressed and on what part.... from a slightly bulging disk or more or calcification hitting the nerve. The best type of doctor to see is a physiatrist who will do an electromyograph (EMG). Its a simple test done in their office with a wand that follows nerve paths. When it goes off it's found a pinched/bruised/dead one. I had been through all kinds of xrays/mri's by other doctors but it was one of those doing an EMG who found the problem. Good luck Paulokes.
Edit: If you have trouble finding one, look for Spine and Sports Injury doctors which many of them handle (mine does) but they also handle any condition affecting skeleton and muscles without being injured... like arthritis.