hamstring pain

by Nancy
(Brooklyn, NY, USA)

I'm a 58-year-old very active female who works out 4 days a week - 2 days spinning & 2 days weight training. Around 3 years ago I did some yoga. During 1 sitting pose with legs straddled far apart, reaching for the right toe & then the left as far & as low as possible, I felt a twinge of pain deep in the back of my upper right thigh - it was a sudden pull that seemed to be a result of overdoing the stretch. The pain never went away. It's not intense but it's always there. Being very flexible, I've always been able, for example, to extend my legs forward when sitting on the floor & reach my nose to my knee & grab my toes, but since then I can do the left but never the right - it just doesn't go beyond a certain point - & I still feel the pull or knot or whatever it is. I neglected it all this time because it never really hampered my life much except in a gym-type situation, which is when I really notice it. Is this a pulled hamstring? Is there anything to be done at this point? Should I see a doctor? have an MRI? pursue some sort of treatment?
Thank you