Why do legs feel lifeless for no reason?

I have been sick with Upper Respiratory Infection for two weeks now and have been on two different antibiotics but during this time my legs have felt just weak. The best way to describe it is like when a car brakes in front of you and it startles you and you have to brake quickly that sensation that you feel thru your legs is exactly how mine feels. It is worse when I am sitting or laying down verses when I am standing. Has anyone every experienced anything like this? There is no pain, swelling, or changing skin color of my legs just that scare sensation that comes and goes.Why do legs feel lifeless for no reason?well everyone body reacts differ to medicine. it could be your lying down and them going to sleep. but if it keeps happening i would call a nurses free info hotline their usually 24 hours they could help you out better.