Reduction in a bitches ability to hold on is normal after a spay surgery - at least for a little while. Spay is complete ovario-hysterectomy. It is more major than most hysterectomy surgeries that humans go through - as they don't just take out the uterus, but the ovaries, horns, tubes, the lot. Spay surgery may be "routine" but it is not minor (on the contrary, this is a major invasive surgery).
A human would normally be hospitalised for at least a week after such an ordeal, and taking things very gently for several weeks afterward
The relevant bit though (in respect of bladder problems), is that the bitch's internal organs - bladder included - take a heck of a beating in the course of a spay, as they're pushed, shoved and clamped aside to allow the surgeon to get at the uterus. And it can often take a bit of time afterward for things to revert to working normally, or full bladder control to be re-established. Quite understandable, if you think about it.
Bottom line there is that the bitch cannot be expected to hang on as she could previously, nor necessarily to "know" terribly long in advance that she needs to go. Or put another way, when she needs to go, it is likely to mean "right now" and she cannot wait.
In the normal course of events, this issue should subside and she should revert to normal levels of control. In the meantime, you should be understanding of her needs, and provide much more frequent and prompt opportunities to potty outside. If things don't return to normal within a week or two now (or sooner if anything else seems amiss), then you should revert to your vet to ensure that there are no other problems.