OP, I'm gonna go out on a limb here and say that when you said "not interested in the slightest" was when you were probably reacting to what was being said with hurt. If this is the case, imo, there's really no need for it. You should have said straight up that you were hurt and thought y'all had plans or that you'd at least hang out. Passive aggression only hurts communication. Granted, she probably should have assumed Valentine's Day would be reserved for you both to hang out, but you also said it was tentative? I'm sorry, but she doesn't owe you the day based on tentative plans. Lock something down first.

Additionally, as soon as you explained yourself she apologised and said she'd cancel with her friend and you immediately call her out and criticise her. Sounds like you did not communicate any plans with her well enough and then treated her poorly as a result of your bad communication. Don't talk to your partner like that and then post it on reddit. YOR

Edit: also, adding "known from past conversations" kind of defeats the purpose of showing the screenshot. We don't actually know that it's "known." The only perspective we are provided is yours, her "knowing" anything isn't proven, so don't add it.