What should I do about the pain in my hamstring?

When I stretch I have severe pain in my right hamstring just above the knee. I usually an pretty flexible, but now can barely stretch my right leg. It doesn't hurt at all unless I am stretching. I run between 2-3 miles each day and it doesn't bother me at all. It has been hurting for at least 3 months and isn't getting any better. When it first happened I did hear a mild popping sound so I took a couple weeks off from working out. Any knowledge of this subject would help. Thanks so much!!