If you've pulled a muscle in your back, the last thing you should do is exercise later today. You should consider laying off any exercise for a couple of days. Give your back a chance to rest and recover. If you try to do Insanity now, you risk making that injury worse than it already is.
You WILL NOT derail any of your efforts at weight loss if you need to take a couple of days off from your exercise routine. In fact, part of a healthy lifestyle is learning to take days off when you're injured. Listen to your body, if your back hurts, you shouldn't exercise until it's fully healed. If you exercise while it's weak, you really do risk hurting yourself.
Take it easy a couple of days. If it still doesn't feel right after a couple of days of ice and rest, give your doctor a call. Back injuries need to be treated with care.