I hear this a lot, as I work with issues of balancing strength and flexibility, and lots of movements to open into that part of the body. Karen is completely right that backing off and reassessing is the way to go. There is pain and there is tension from muscles that are so tight they have difficulty with normal rom. A single word can have many meanings, and helping them both to figure out what they are feeling and to communicate that, that is taking some time to have them describe what they are feeling and when is very helpful. (not just for you, but for them to develop the ability to feel it and avoid moving in ways that are problematic or unsafe)
I agree with everything Karin suggests. Massage, rolling, and daily balancing rom exercises all have their place. Doing some postural assessment to see if there is imbalance is particularly helpful. The hamstrings neither exist nor function in isolation. For example, I find that if someone has a limb length discrepancy I may open the area in question, but daily motion will keep bringing it back. In these cases, and if the discomfort began after a specific incident (i.e. if it is acute rather than chronic) I will generally suggest they connect with their medical advisor or a pt.