pain in lower left leg

I am a 22 year old male, I started running after being very lethargic and having a poor diet for about 4 years.  I changed my diet and my lifestyle, but after running for about a month I started developing a pain in my lower left leg.  I'm using a good pair of running shoes and taking time to stretch.  I was not concerned at first and took a few days off, that resumed running, but the pain got worse.  Now I have stopped running (but continued other work outs and maintaining a healthy diet) except once a week, where I'm required to run for weekly Marine Corps preparation training.  I'm not sure what's wrong with my leg.  I saw a sports doctor during a free day, he took a quick xray suggested it was most likely shin splints and told me to take a week~10 days off.  It has been over 3 weeks sense than and the pain is still there.

The pain is very sharp and centered on one point.  If feels like it's coming from the middle of the leg, around the bone area, but I can't be sure.  It doesn't hurt at rest, only when applying or releasing pressure on the leg.  It has gotten better to wear I can't feel much pain (if any, some days) when walking, but the moment I try to run, the pain becomes acute with every step on the left leg.  I used to be active in High School and have had muscle pain and such before (though admittedly never shin splints) and this just doesn't feel like any muscle pain I know of.  If it was shin splints, I would have expected it to go away in 3+ weeks.  It has me really concerned, and I want to know if this is worth going to another doctor to get another opinion.  I want to know exactly whats going on so I can take care of it and heal it properly to get it dealt with as fast as possible.

Thank you for any help and advice.