Are you able to maintain that even when you really like the girl? I do the same but one time it didn't work bc I caught feelz. I tried so hard you just withdraw and keep playing it cool but it felt like a sham so I tanked it.
How did you tank it exactly? Playing it "cool" is such a broad term. I tried playing it cool with this last girl I dated and I think all it did was postpone the inevitable. My definition of playing it cool is just giving them their space, not checking in on them, not showing any anxiety you have about them not reaching out to you, waiting for them to set dates when they seem to be cooling down, not double texting, being ok with not seeing them for a couple weeks, that kind of thing. But like your situation, because this girl was rare, I caught feelings and it made it so unpleasant for me even though I could keep being "cool". She'd do things like totally ignore a nude pic I'd send her, then text me 12 hours later with something completely unrelated like a description of what she had for lunch. Or she'd ignore my request for a date for 12-24 hours, yet I'd see her posting on FB. I had quite an internal battle over whether this was low IL, sh*t testing, or just how things are with this girl and that I should learn to not waste any time and energy on it and instead focus on enjoying her company and s*x when we are together and call it good. So I did
really well in showing zero reaction to this kind of sh*t - nearly perfect actually. It didn't cause me to get needy, and ultimately she would ALWAYS reach out to me and set dates. However, just because I gave her a long leash and played it cool outwardly didn't mean I wasn't anxious inside. I was *miserable*, in the way only a girl you are really into can make you. So that's why I ended up "dumping" her. I could keep playing it cool, but it wasn't how I felt internally so it was just better for me to get out.
TL;DR: If you have to keep playing it cool past 2-3 months, she isn't that into you, and playing the game will only postpone the inevitable.
To OP: I wouldn't worry about an isolated incident like this from a GF of over 2 years. It happens. Are there any other signs recently? I would keep an eye open and maybe this is a warning for you to be on your toes - maybe pull back a bit and see how she reacts.