Need advice

I don't plan on announcing until I'm 14 weeks (personal choice). Today I had someone post on my Instagram telling me congrats.... (Someone my MIL told). It's fine that she knows, but I'm not ready for it to be on Facebook/Instagram yet. What is a nice way to tell her that we are trying to keep it a secret for a while? Every time I try to write her a message, it comes out a bit rude (probably because I'm a little upset over it.).