Homeopathic arnica tablets can reduce the onset of bruising, but they need to be taken few hours apart from each dose, and as soon as possible after spanking. To put some arnica cream on soothes the skin and helps with healing too. If your bottom is just red, I wouldn't worry too much tough. It's more tricky if it turns into bruising. I used to put make up on my bum when that happened, just to take the edge of that bluish or greenlike tone.
If everything fails you could propably say something like you tried a bit of spanking but got carried away last night and didn't imagine you'd have any marks left - in case any new client looks puzzled. I would try to get it across as the most natural thing in the world, so you don't give out the impression you were beaten non consensually or have an abusive pimp/boyfriend behind you.
Hope it goes away anyway. A few years ago I used to do spanking and some amateur modelling aside. It was a nightmare to juggle these two jobs, as I'd never know if I would to bruise or not and how long it would take to recover. I felt I had to play the devil's advocate when agreeing to bookings and just hoping no marks would appear on that video/photoshoot. The problem is that cameras tend to pick up more on differences in your skintone (where make up has been applied) etc. Aaargghh!