Hi D. -
I used to be a runner, stopped when I had my children, and recently started again. I thought I had really bad shin splints and they wouldn't go away. It kept getting worse in my right leg. Some days it would hurt so bad I couldn't even knell or walk. Finally I went to the doctor and found out I had a stress fracture. I have still worked out, but stopped running and walking and do mostly core and upper body. For cardio I cycle or use the elliptical, but I had to make sure I used my right heel to keep from putting pressure on my fracture.
It's now been about 3 weeks, and after icing it daily and about 100 advil - it is pretty much healed up. I haven't started running again, my doctor advised me to ease back into very slowly. Honestly though, I have been able to vary my routine so much that I don't miss running as much and I still work out just as hard 4-5 times a week.
Hopefully it is nothing more, but they did do x-rays, so I would go to the doctor and have it checked out.
Good Luck! L.:)