Thanks for coming and sharing this. Based on my discussions with other monogamous folks faced with something similar, your fears are quite typical.
If your partner truly is poly, then you will find out afterwards that your fears were utterly unfounded.
Have you agreed on talking to each other during the weekend for you to get status updates and stuff? Or would you rather not hear until afterwards?
What you are going through right now is far from unusual. Hopefully it will be a small consolation to know that it eases over time.
"Listen, or your tongue will make you deaf." - Native American Proverb