I wouldn't worry about it then
If she seemed ok then she either didn't see you masturbating or she did and she doesn't mind.
But don't be too obvious about it, whatever you do. If it turns out she just didn't see you before, and if you do it again and she does, she might not like it and you could really get in to a lot of trouble.
Don't mention it to her (keep flirting with her if you want though!) and don't be obvious about it. Wait and see if she brings it up.