don't hide what you do. be a man, damnit. simply tell her that you took a "hormonal supplement."
she'll say, "huh?"
and you'll say, "i took a supplement to help me be more fit. but there is a consequence, though. it has squashed my libido for a few days. i should be on point, within the week, though. it's science, baby."
and if she asks about the supplement, aka, "are you taking steroids?"
be honest and say, "well, no, but the pill does convert to 'something like a steroid' (say that, it'll ease the blow), though. heck, it made me feel pretty good, but in retrospect, i won't likely pop them again anytime soon. i don't particularly like the tradeoff of even one week of my libido for all the fitness in the world. but i come to find out that it IS a side effect of the supplement. anyways, i'd like to go down on you now. twice."
the advice from the others is to lie. never lie if you do not have to.