Dear yog enthusiast,
I welcome you for your follow up as well as thank you for very nicely elaborated & self-explanatory reply.
You are practising the pranayama absolutely right (starting from left nostril and coming back to left nostril breathing through right nostril breathing). I recommend little addition (you shall be doing) - first exhale from left nostril and then start the pranayama as you mentioned. It has no adverse effects subject to the condition that you practice without tiredness and overstrain. Asanam sthiram sukham meaning thereby that posture is that which is steady and comfortable. If any of these two conditions are not fulfilled, the whole practice becomes erratic.
However, to find out the cause, I may recommend you to just leave the pranayama and sit for the same duration as you use to spend during the pranayama. Just observe your breath; watch how the breath enters in and through the nostrils while touching the walls of the nostrils and how the breath comes out through the nostrils while touching the walls of nostrils. Also feel the temperature of inhaled breath and exhaled breath. Feel the relatively coolness of the incoming breath and relatively warmness of the outgoing breath. (observe means just observe without interferring with the rhythm of the breath; you just observe the rhythm even.
Kindly follow the above practice in place of pranayama in question for 2- 3 days, and then follow up with the observations with regard to twitching in the lower limbs during the suggested practice. Even you can follow up with observation for one day practice.
Be Happy and peaceful!