Question on cramping

I've been practicing this type of stretching for a couple of weeks now and can already see the enormous potential. However, I've been getting cramps in one leg on the posterior side. Mostly it's been the posterior tibia, but there has also been some hamstring cramping and plantar (sole of the foot) cramping. Interestingly the cramping does not happen during my stretching sessions, it's either at night or toward the end of my swimming workouts. Cramping has never been a problem for me, and now I've had three major bouts in five days.
Any ideas?