Question about a stiff back and lifting weights while sore....

[RoyalMarine8]

I've been using a bag of weights to do push ups ( 11 reps ) I put the bag high up so that its just below my neck and on my shoulder blades ( makes it easier on my back and harder on my chest ) so far this bag push up thing has gave me more size than going to the gym and using the free weights and machines... But Its making my lower back painful when I bend over to pick stuff up is this the bag or just bad technique?

And is it bad to workout a muscle while it is still sore from the last workout? Say if I worked chest and triceps on Monday and by Friday I wanted to work them out again but they were still stiff should I wait another day or just work out because I'd of had 4 days for it to recover

[comps]

There is a good chance you are just sore.