I would let your spine doc know. It's possible to be a complication from your lumbar surgery. I know pre op for lumbar patients they ask if you have had any loss of function of bladder or bowel. It's a sign you need your surgery sooner rather than later. It doesn't seem far fetched to me that it could be related to the surgery. I've had three lumbar surgeries last one being a fusion and I was asked that question each time before I had surgery.
When the vaginal wall isn't very strong it can make it easy to have these sort of accidents. Having children isn't the only thing that will make the wall weaker though- weight, surgery, age, and definitely genetics do this as well. Pretty much anything that will put pressure on the bladder.
I strongly suggest looking into kegals. These are exercises that will strengthen the vaginal wall and will make these accidence hopefully disappear completely or at least make them easier to deal with. To do these all you need to do is use the muscles that hold your urine; tighten them for 30 second and release them. This can be done anytime of the day, and overtime will really make a lot of difference.
However, it's possible that surgery or something else could be causing this. If you're from a family with weaker bladders (for example my mom and grandma both used to "laugh till tears ran down their leg") you may be at more risk. Also if you're overweight it could help to try to loose weight. If you don't have any such obvious causes, perhaps you should talk to your dr to see if there is anything else you can do that will help.