My activity was 30 and I was Jr.Member this morning. I'm sure of that.
Now my activity has dropped to 29 and I'm newbie again. I did not delete any of my posts, or none of my posts were deleted by a moderator.
Why

Let me quote in for you:
If you've made a lot of posts in a thread and it gets trashed then your post count will go down. Don't post in unsubstantial threads that are liable to be deleted if you're worried about this.
I'm sure that the moderator deleted at least one of your post causing your activity to go down to newbie account again.