it is possible that my activity 'is being increased day by day?
I do not remember what I was, but I certainly increased twice yesterday ...
Activity is determined by a combination of 2 things.
1. How many activity periods you've posted in. If you have posted in 3 activity periods, then your activity is 42. However, onto point 2.
2. Your activity cannot exceed your post count. The potential activity is never lost. The post count just needs to be increased.
If you have 3000 posts and 1000 activity, then you only need to make 1 post every activity period to increase it by 14.
If you have 500 activity, and 200 posts, then your activity is capped by your post count, your activity will rise with your post count until all potential activity has been exhausted (this is your scenario, though I don't know your exact activity).
This is all you need to know about activity.