My Colleague's Coffee Is Now My Problem

I’ve been working at this company for five years now as a software engineer. I’m quite the introvert and prefer to keep my interactions limited, so naturally, I don’t really socialize much beyond basic workplace pleasantries. That’s not to say I dislike anyone specifically, but rather, it just isn’t something that interests me personally. Recently, however, one of my colleagues, Rachel, has taken a particular interest in me, which is quite out of character for her too. She always comes into the office early and brews herself a large pot of coffee each morning. I noticed she began bringing some of it over to my desk around the same time I started coming in earlier than usual. At first, I appreciated the gesture, caffeine is essential when starting the workday early for me too. However, as days went on, Rachel’s visits became more and more frequent and her coffee wasn’t always great quality; sometimes it was weak or burnt tasting. Still, I felt obliged to drink it since she made such a fuss about bringing it over. One day, out of frustration with my sleep being cut short again due to starting work early, I found myself pouring most of the coffee down the drain and refilling her thermos with plain water. When she came back, she was all smiles and praised the strong taste, oblivious that her cup contained nothing but what I had refilled it with. I’ve been agonizing over this since then; am I the asshole for doing something underhanded like that to make my mornings a bit more tolerable? It certainly felt better in the moment, but the guilt hasn’t left me. What should someone do in such a situation?