You can only assume that she did not assume if you further assume that it did not occur to Mary to ask a very legitimate question: "Why appear to me now to tell me this?". You must assume that Mary did not think this pregnancy was imminent. I think she did and she was right. We know that she was pregnant within three months (three months being the longest time possible) of the annunciation because of her visit to Elizabeth.
The words of the angel never make any reference to any time frame, so no, I do not have to assume that. Three months is a reasonable timeframe, which we know from the other scripture. Even if Mary took the words to mean that she will become pregnant sooner than the anticipated three months, her response still makes no sense, a natural response would be "Will the wedding date be moved forward? I can't wait to tell Joseph, etc."