If you've got really bad hamstring pain, or any really bad pain for that matter, this definitely isn't the first place you should come for answers. You could initially try WebMD's symptom checker (I've found it useful) for a quick idea of what it could be, but really you should be asking a doctor.
It could be any number of things...depending on what kind of pain it is, what causes it, how sudden or gradual it comes on, if it causes a limp/change in your running form etc etc...
I've had my hamstring completely rebuilt and reattatched, so yes I've had a hamstring problem lol, but it was also in conjunction with tearing my PCL, LCL, MCL, and meniscus all in one clusterfvck of an injury.
But seriously you should ask someone who's qualified to give you an accurate answer and who can examine you.
