Nothing really happened just feel like it's been 2 weeks since my last activity update. Would be nice to know how the forum calculates it. If the starting point is the registration date, then my points should be added on the 30th of every month, but that didn't happen. (I'm quite sure my points are exactly the same for over a week). It's not much of a problem I can wait a bit more.
As I got it, the day activity increases doesn't depend on your registration date. As said before, it increases automatically for all at the same time...

It's not. Just wait for the activity to update and then you'll be able to work it out.