It sounds to me like plan B was the morning after pill? (sounds like it, at any rate, didn't ckeck out your link
)
I used those on two (!) occasions with my previous GF, and they're basically hormone bombs, according to my doctor - i.e. a slight change in the menstrual cycle is hardly out of the question (also, the fluids were different than usual the period after using the pill), and it could also explain the pain...