W text me asking if we were still meeting for a drink after work. I tell her that I am UNAVAILABLE and leave it at that. Telling her you are busy at work tells her you cannot make it, not that you choose not to make it. Also it is a lie, do you like not telling the truth? Just say you cannot, make her wonder why. What could he be doing that is more attractive than spending time with me? He always wanted to be with me, what has changed? Oh dear, perhaps he is turning the tables on me?
Plus I would NOT have offered the alternative time. You are still making yourself a plan B if OM is around. I do not know what inference you took from the "I have plans" response, but obviously something more attractive than spending time with you. Stop being a dorrmat.