caught my partner experimenting

so i was chillin one night and my phone buzzes with a text from my long-time partner who's in an open relationship. we've been together for like two years, and even though we're non-monogamous, i feel pretty possessive sometimes. anyway this time they ask me if its cool if they try something new with another person - they want to do a bdsm scene where they're subbed. it sounds hot but also kinda freaks me out. still says yes cuz that's the deal in our relationship but starts feeling really insecure and unsure like maybe i don't know them as well as i thought? we set up a time when this other person comes over, and i end up hanging out too. its weird watching them be submissive - they never act that way with me. after it happens, there's an awkward silence for a minute until my partner looks at me with tears in their eyes and says how much they enjoyed it but also felt really vulnerable and exposed afterwards. it made me realize i need to talk more about our boundaries and what we're comfortable with instead of just going along with things cuz its the arrangement. still trying to wrap head around all these new feelings and dynamics.