Do you have any deleted posts last week? If one of your posts in 2-week intervals is deleted and you don't have any other post in that period you lost your 14 activity points.
Certainly not this one, as OP have 289 posts as of this moment. I haven't heard such an activity decrease in the past in this forum, and I know that the only possible way that it can be done is via nuking or deleting posts of a certain user up until it would go lower than the activity of the user.