I hope you will be ok bro. Dead lifts are not an exercise you want to go for "one more rep" on. When you feel you have done your max amounts of reps, you need to stop. Is there any bruising on your back? If not, then you probably didn't tear a muscle. I don't think you slipped a disk either, because if you had you would be in too much agony to even type this message! I suspect you just pulled a muscle. If this is the case you need to rest this muscle until it fully heals. Good luck bro.