As you were doing a lots of sit ups and leg lifts may be your nerve
get streched. That only can be the reason can think of. So for any nerve iinjury the treatment rest is the best treatment. Sleep in proper position. Take some painkillers with muscle relaxant
to reduce pain and it will help you to have proper sleep. Also try to contact some physiotherapist and do some passive exercises mentioned. I hope it will help but this takes little time to recover.