Dear Juliag,
It could be partially that: you are not used to using those muscles like that; have not learned to relax the legs a bit more; and that channels are opening due to the challenge placed on them in this posture.
Warm, tingly, or aching feet is pretty common with new students and the active exercises. If you need to, place a chair behind you, and without having to miss much of a beat, you can continue from a seated position.
Best,
Shawn
