Note that Iron worker specifically said your LOWER back. If you want to get very technical, rounding of the upper back is perfectly fine, and can even help you lift heavier at much higher weights, but I would not recommend it especially if you workout alone because you won't know that you're potentially doing something wrong until you hurt yourself. Again, the lower back must always remain locked.
To answer your question, if you have a hard time bending down, you either hurt your back or you're experiencing DOMS in your lower back and/or hips/glutes. If it HURTS, as in pain, then you've probably injured yourself and should stay away from back exercises for a while. If it's just soreness, then there shouldn't be a serious problem. You could possibly be over training or not getting enough calories.