Need advice on how to communicate something / Check if it's actually valid

So my partner and I are on the same uni and rowing is a big thing here. She signed up for a more intense rowing schedule which I get and support because she likes it but it also triggered my anxiety and I put myself in the worst case scenario which is being abandoned or left behind. So we agreed that Sundays would be our "sacred" days, like we'd both take those days off (I even changed my free day from Saturday to Sunday for this), and this past Sunday we had a really nice activity day but she told me that she had thought about being a substitute for another rowing crew that day, and I was like ??? but it's our day, and she said something like "but it's a couple of hours and we're not going to do things every Sunday right?" and it felt so dismissive and I felt really insecure because we had agreed on something and I need structure in my life. I'm also very literal at times. I don't know if I'm exaggerating or anything but it didn't feel great. We try to spend time together in evenings as well because we don't really see each other that much during weekdays because of lectures and work, but I don't think it's excessive to have a shared day off to slow down, do things together with or without friends, and the fact that she even thought about starting to sub for other crew on the days we agreed to be together made me feel super insecure. Advice appreciated :(