Ideally one could reply to this with, "who cares what they think?" but we all know this isn't always the easiest to consider in our lovely modern society.
Typically speaking, there's nothing wrong with sending yourself flowers, especially on Valentines'. If nothing else, as a flower-fan this is one of the best times of the season for specials, discounts, and free extras with orders! You *could* simply go purchase them at wholesale florists for a lot cheaper and get just as good an effect, however, if you're leaning more towards smiply having fresh flowers for the ocassion.
With regards to clever excuse, there's nothing wrong with just smiling and saying "it's a secret." You could simply tell the truth - that you were in the mood and wanted to take advantage of the specials (should this be the case). If you're overly concerned with someone else's interest of your flowers, well...it's not necessarily their business if you don't want to admit that you just wanted a treat for the big day, anyways. :)