If it hurts, maybe do knees for now.... I'm not an expert but it should not hurt. If it does then you might want to skip it or just go to your knees and build your strength first that way. I just saw a healer who works a lot with body alignment, awareness, posture, etc. We talked a lot about yoga. I guess a lot of that power yoga that incorporates those sorts of poses can actually aggravate, not help, your body. After speaking to him I am going to focus on more gentle stretching and less of the vinyasa flow myself for now. So you might consider that yourself...certainly when the ancient yogis did yoga they were not doing plank and downdog over and over again.
You are not tucking your pelvis into your ribcage; you are likely arching your back when doing it. Try a "humping motion" with your pelvis while on your knees; put a stick on your back with a mirror on the side to make sure lower back is level and you don't have a "gap" in your lower back.