I assume that you don't want her to know anything and that you can't be out at all. Did she see your account name, or just that you were on this site? How clearly did she see your profile? You can just say that you're curious about LGBT+ issues, or that you're an ally and just want to learn more. If she checks, see if you can change your gender/orientation/out status information (the stuff shown on the left of your post) temporarily to something fake. You can go check/change your privacy in profile settings too so that people who don't have EC/or who aren't your friends can't see some info about you on your profile page- but I'm fairly sure that anyone, regardless of account or not, can see your immediate information (eg username, avatar, immediate information on the left hand side of your posts) if they come onto this site and read the threads. (Someone correct me if I'm wrong on this or if there's another option to hide this) If push comes to shove and if it really is urgent to hide everything, and there's no other way, I'm pretty sure you can request a username change.
Hi there! Mackenziesr provided some pretty good suggestions. If or should you change your mind, feel free to create a thread in Ask The Staff with a new username, and a Community Facilitator or Admin will change it for you.