I know it's easier said then done, but the best thing is to just straight out tell them that you smoke and the pros of smoking and how it's not nearly as bad as most of the other drugs that are out there. lying only makes things so much worse, it's such a relief when you tell the truth afterwards. if they decide not to talk to you for a while, well let them be their problem.
I'm kind of in the same situation now with my parents. they didn't catch me, but they found out after the first couple times I smoked. my parents seemed to be cool with it and told me not to over do it. I assured them I wouldn't and I haven't. That day, we had a long conversation about pot and the how my dad used to toke up until he whitied lol.
but now, things have gotten progressively uglier. my mom found my plant and have caught me after I have smoked with a friend a couple times, never in the act (yeah, i know that makes me sound under 18, but I'm still living with my parents because I still have one more year of high school to finish, I got left back a year lol). she also found the RIU page up on the computer and she read damn near every post on here. it's horrible and pretty embarrassing. she thinks I'm some sort of pothead now and that I smoke ALL the time. truth is, I only smoke every two weeks or so. i told her that, but she still insists that I don't smoke even though she does. fucking hypocrite...I understand her reasoning to an extent (her reasoning being that it will ruin my life and all this other bullshit...I honestly don't see how she smokes pot and is still sucked in by the false accusations pot is given). I can't argue with her without my dad butting in and telling me not to back talk to my mother or threatening to beat the shit out of me.....you know, I'm getting too into this. I'm really sorry.
point is, you should just tell them. you may get some snide remarks, but it could be SO much worse. they know that you're the same person after smoking, it just might be a little awkward the first hour or so they know this little bit of info about you.