My gf and I are really into cumplay and humiliation, and we would like to try one new idea we've been thinking of for a while.

Essentially, I want to cum in her panties and them make her wear them out in public afterwards. This is going to sound amusing paranoid, but she's on the pill and I'm concerned about pregnancy. The reason is that she's on the pill but her period isn't regulated. So, there are two questions, really. I'm no doctor, but it seems that the pill not regulating her period might mean its not working. Does anyone know anything about that?

Also, even if it isn't working, what are the odds of sperm managing to travel into her vagina and all the way to the egg from inside her panties.

Thanks in advance!