Hamstrings are funny when it comes to "tightness". Some people actually have tight hamstrings, but often the hamstrings feel tight, when in fact they are not. Simple test for yourself: lie on your back with both knees straight, and toes pointing toward the ceiling. Lift one leg up as high as you can, keeping both knees straight. If your leg gets anywhere near 90 degrees, your hamstrings are not tight. If you do this test and your leg is barely 45 degrees, then yes, you have tight hamstrings, and stretching and rolling is probably wise.
So what's making your hamstrings feel tight if they aren't actually? My guess would be tight hip flexors. Maybe coupled with weak core. The hip flexors can pull your entire pelvis forward, which puts your hamstrings on stretch when they are relaxed. When you then use them, they get stretched even more. Another test for this: when you stretch your hamstrings, do they feel better or worse after? If they feel worse, odds are you're stretching an already stretched muscle. Stretch the hip flexors instead and see how you feel. Now this is unlikely to stick if you don't also strengthen the core, as that's what you need to hold that position.
If you were experiencing this from running, I'd throw in weak glutes as a possibility, as hamstring issues often result from the hammies trying to do the work of poorly functioning glutes. But from what you described, that doesn't appear to be the case.
Since you're getting cramping, diet is definitely something to look into as well.
Considering it could be a few things, I'd second the suggestion above to get help from a health care practitioner. Know a good sports med doc or manual therapist?
PS - Thanks for the kind words, Adie! :)