Alternate nostril breathing / pain in muscles of thigh

I do pranayama exercises regularly, including Nadi Shodhana (alternate nostril breathing). My rhythm for this is 4 seconds for inhale, 16 seconds for breath hold, and 8 seconds for exhale. I have been practicing for a few months now, but I can't manage more than 10 rounds. After about 5 rounds I get pain in the muscles on the front of the thigh. If I then breathe normally, the pain disappears very quickly. I have tried different sitting postures (Sukhasana and Swastikasana). There is no difference, the pain comes after a few rounds. With other pranayama exercises, such as Kapalabhati, or when meditating, there is no pain in the thigh. What can I do to reduce the pain?