Weight Lifting after an injury?

I just recently got over my back injury. before the injury i went to the gym alot to workout, but after i got injured i had to take 3-4 weeks off of all exercising. I love to workout and im glad i can do it again but when i went to the gym today i couldn't lift as much as i could before, i use to be able to bench 135 for my warm up and then do heavier weights for my actual sets and today i could barely do 135 6 reps. i understand after not lifting for a couple weeks id lose results that i had worked so hard to get but not like this..... is this normal? how long does it take for me to be able to lift my previous regular weight? what would be the best way for me to build back up?? please help me out , thank you