While Gabrielle gets on with digging down into the issue, I wanted to add something as I have experienced exactly this issue in my early practice.
I was following the instructor’s commands (no, not Gabrielle’s ha ha) ... and really keen to get my head to the floor (read “ego” in my case - I’m sure that’s not you Jacquie
) - so was using a very very wide stance that simply was putting too much tension into the adductors (inner thigh muscles that pull the legs in) and hamstrings.
I realize that for you, you were already at the floor with your forehead - however, as you gain strength with your practice you have probably been able to push harder to straighten the legs; apply more effort with your (now stronger) arms etc which could be why it is only a “new thing” - the issue may have always been there in the way you are practicing the pose, but only manifesting itself as pain now.
Anyway ...
Occasionally I’d get close or even touch - then what would happen is that I’d be trying harder and harder ... then suddenly something would feel like it had “snapped”, I’d say “ouch” .... and then the next pose (Triangle) would be impossible because of the pain.
Occasionally I would miss one or two days practice because of the pain.
Clearly this is not yoga!
After a while, I realized my body was being pushed “beyond its design capabilities” and reverted to a less wide stance until I gained sufficient flexibility - but working even harder on the back stretch. For me this took around 9 months to then get my head to the floor “at will” with none of the previous over-stretching pain - and when I did eventually get there, my posture was soooo much more satisfying.
There is a way to mitigate this in the meantime and prevent injury - in fact Gabrielle who just walked into my office has just reminded me of a very relevant article she wrote right here:
http://www.hotyogadoctor.com/index.php/site/links/opening-up-your-hamstrings-with-hot-yoga/
In the meantime I’m guessing you may need a short break just to heal the soreness - yoga stretching should be intense to the point of “almost pain” ... but real searing pain is your body telling you “ooops, too far!”. Trust me, I’ve done it that many times! (I think it’s a man-thing to try too hard in Yoga)
Hope that helps at least knowing you are not alone!
I too shall eagerly await Gabrielle’s wisdom ...
Robert