What Causes Severe Leg Pain?

I am 45 yr old female had meiners disese and inner ear surgery in 95 and 99. in 2005 was diagnosied with valley fever and asthma,n then in 2007 fibromyalgia,Now the Dr is uncertain. Question is if the Dr is uncertain what is causing my severve leg pain then why do A doppler test? my leg isnt red or swollen. I Understand the bone scan with out reason also because of very mild disc degernerative disease "common at my age' I just have bad leg pain dull like pain almost all the time at times a twitch and ,at times a 'charlie horse' on one leg or another lower as in my muscle areas in my leg what else can a Dr be looking for,? and why would a Dr automatically decribed 80 mg predisone for 6 days? which I only been on for 2 days and I can see and feel a difference? THANKS'