I agree wholeheartedly with Rip about not using a mirror. I think it gives you false spatial information. I wouldn't do it. Short of having someone with a practiced eye help you you need to be able to feel the correct position from a proprioception standpoint. I think many people may be looking at what their squat looks like as compared to someone else as well, and unless you body is an exact copy of theirs, your squat will not match perfectly. You need to have a sense of how your body moves. Visual cues not only are not the way to do this they actually separate you from having an awareness of your true spatial positioning. I think this is known as kinesthetic awareness.
Drallen, sounds like quite a strain. Can you pinpoint an area with your fingers that is more tender? Just becasue you feel the pain in a wide area doesn't mean it's a big wide strain
When the pain subsides try to massage and stretch the area gently. Then you can find those things you can do that don't cause pain and work your way back in. You have to let pain be your guide. But when dealing with a strain like that do expect some discomfort. You do not want to just rest your back for weeks and weeks though. That would be the worse thing to do.
Let us know how you're doing pain wise in a few days.