From a mom's perspective: don't assume she's stupid, don't lie to her. she can find out what it is if she wants to. Print out some info about what your doing, show her your log book, whatever proof you can give her that you are using responsibly and you know what you're doing (even though you may not).
She's probably freaked your on herion or something. When she calms down, tell her your reasons, tell her everything. Try an educate her as best you can.
There is a bright side.... you may be able to move out now before you get married!