We usually recommend that they are used right foot first, as Wrenn accurately suggested, then left. Continue alternating until you get comfortable and in a good detox rhythm. At this point in the detox process you can start two feet if you feel you are ready.
What you experienced was most probably a slight "healing crisis", and although it is usually a sign that the cleanse is working, it can be somewhat unsettling and avoided most of the time.
Congratulations and keep getting healthy.