um.. my advice: lie. lie like you've never lied before.
While getting all the info about e is a good idea, most likely your mom wont buy it. if she does some research on her own, even if she just asks around, she'll find out that not every pill is alike and that you can't really tell what's in the exact pill you're taking.
Hopefully your mom is an ex hippie. And hopefully, you don't have a bad drug track record with her.
If this is the case, tell her you were curious. Tell her, and this is important) that none of your friends do it. (REMEMBER: your mom will forgive you. you're her child. However, your mom WILL NOT forgive your friends.) Tell ehr you were curious. you wanetd to try it. So you asked someone who graduated from your school (also important - don't pin blame!!! It will come back to you - guaranteed).
Anyway, tell her you did it once because you wanted to try it so you would know what you were missing. It was curioisity. you did it by yourself. And you HATED IT (important!!) You hated it so much.
Go on about how you hated the feeling of not having control (brownie points: your mom will think you're straight edge) and how you never want to feel that way again.
Also, if you don't make a big deal out of it, your parents wont either. if you get all nervous and freak out, they'll think they have something to worry about.
I sympathize with you. I've been in your position before. A while ago my parnets found my herb paraphanalia. Being pot heads, amongst other things, themselves, I didn't think I had anything to worry about. however, my parents seemed particularaly concerned. Apparantly I had some xanex in that bag which i had forgotten about and it happened to have been crushed into a fine white powder (white xanex? I don't know..). Well, as you can imagine, they were none too pleased. But since it wasn't coke (and oh did I find out some interesting things about my parents through that accusation "how do you even know what coke looks like, dad?...") I didn't act like it was. I challenged them to take it to the lab and eventually everything was dropped.
But you have to make sure you don't accuse your mom of invading your privacy or something. she'll just think you have more to hide. Anyway, hope I helped some. best of luck!! Don't worry too much - worrying solves nothing! PLAN!