Read this post if you like to learn more about the activity system it does increase but only 1 time each 14 days
https://bitcointalksearch.org/topic/forum-rankspositionsbadges-what-do-those-shiny-coins-under-my-name-mean-178608
checkout this nice spreadsheet : https://docs.google.com/spreadsheets/d/12saLhlUoqIdairxzuSPu6EYGrt7FN2lOstO1yDjCEbA/edit?pref=2&pli=1
or this topic
https://bitcointalksearch.org/topic/til-activity-does-not-work-on-precisely-2-week-intervals-interval-timings-582736
by the way why my activity decrease it was 59 before now it turned to 56
Your activity can decrease if you or a moderator delete some of your posts this can affect your activity as well!