Re: l5 s1 back problem
Because you weren't doing a lot of activity right after the operation, your muscles are probably a little deconditioned, so it's easier to pull something. Doctors forget that when they say "let pain be your guide", that the pain doesn't kick in until you've already overdone it. It's better to ease into activities, take frequent breaks, and work back up to normal over a period of a few weeks.
Follow up with your doctor -- while you are waiting for a call or appointment, be sure to ice the area, use Advil/Aleve if safe for you, and try not to worry too much. Bending is not real great for your back in general, and since you have already had one injury, you might try to find ways to re-engineer parts of your life/work to avoid bending as much as possible.