Your hamstrings are not tight

In my experience, most people who claim to have tight hamstrings actually have an unrealistic expectation of how flexible they are supposed to be. As a coach, I am looking to get my athletes and clients to have about 100 to 110 degrees of hip flexion with a straight leg. That would take the leg slightly past vertical while lying on your back. Now, having more range than that can be good or bad, depending on your stability through the range of motion.

Stability, and I don't mean being able to do blindfolded single leg squats on a bosu ball while doing the YMCA, is arguably more important that static flexibility. If you are unable to stay 100% rock solid stable throughout a range of motion, you shouldn't be using that range, you'll wind up injuring yourself at some point. You'll need that flexibility to get through the range, but you need to be able to stay 'tight' or stable as well.

Static flexibility refers to the ability to lengthen a specific muscle or muscle group in a slow and controlled manner, and holding the stretch for longer than 30 seconds.

Dynamic flexibility refers to the ability to quickly get through a range of motion, such as a high kick in judo or your ability to do a full squat.

Stability refers to the ability to keep tension throughout your body, so that the prime movers for the given exercise can be used effectively. For example, if you're doing a squat and I try to push you over, I shouldn't be able to move you at all.

Static and dynamic flexibility are both great, assuming that you can stay stable throughout your range of motion. If you can't, then you need to get out of the stretch, regroup, reset, and retry. Before you start any stretch at all, make sure you've got your belly tight. You don't need to squeeze as if Chuck Norris was going to roundhouse kick you, 20-30% tension will be plenty. 

Here's some homework for you: spend 10 minutes per day stretching something other than your hamstrings.  Try stretching your hip flexors, glutes, adductors, calves, pecs, lats and triceps. Remember, belly tight, find the range that's tight, spend your time there, and reap the rewards of improved flexibility.