There are a couple things you can do. You can have a different type of show that is maybe more nonnude and teasy or games that you don't need to take your underwear off for. You could also use a tampon, snip the tail short, and then tuck it in so it's unseen. Menstrual cups are another option. I personally use a combo of different show type and a Diva menstrual cup. Of course, if the model is cramping bad or feeling really off, there isn't any reason she can't hop on Twitter and let her followers know she won't be getting on because it's shark week. If she's worried about meeting her monthly goals and needs the night or week off, she could always ask for offline tips.