Yes thats true, I think I'll skip this particular posture, I practice it some-time it does cause a lot of pain.
Oof. It shouldn't (duh, thanks Sherlock.). I would definitely stop doing it until you figure out what's going wrong...
Do you have a teacher, or are you learning alone?
Either way, I would highly recommend
Light on Yoga by BKS Iyengar. It has some of the best descriptions and highly detailed photos of almost every posture, including the intermediary steps to get into them, and he was an alignment freak, so it is very good for the basics.
Having looked a little closer, it looks like your problem may be originating with your chinlock. It has to be quite firm in order for the spine to straighten properly, which would alleviate any pain you may be feeling in your back.
I would try doing the posture without the mat and see if it's any easier.