Maybe there's something about the shape of the SEAT of your trainer that is pushing against the main nerve/artery of your leg, lessening the circulation to that leg just a bit- enough for it to cause numbness after a while. Just like when we sit on our foot and it goes numb. It could be subtle and cause the effect slowly. Can you put a different seat on it to try? At the very least, stop riding the trainer for a week to see if that is the culprit rather than something wrong with you. Seems a bit excessive to me to schedule MRI's and such if it's just something putting pressure on a nerve or vein.