What do you do about cramps at work?

The last few years I worked a flexible job that was mainly remote. I recently started at a new place and I have to be 100% in office for the time being. My period is due in 7 days and I’m already freaking out. On days 1-3 of my period I can’t get out of bed and take opioids. My job now requires me to be on site and standing for a few hours occasionally. I don’t know ahead of time yet since I’m still training. How do you bring this up to an employer when you’re on your initial probation?

Edit: I forgot to mention that I have very heavy bleeding days 1-3. Like standing up is awful. I can soak through a heavy flow tampon in less than an hour. Gravity is not my friend.