Credits going up?

http://dollchitchat.forumotion.co.uk/

 Hi new here and have a question,(hope i'm in the right place) why do the credits keep going up?
One day you purchase 500mb for 500 credits and next day it's 500mb for 938 credits.
Seems to be going up by the day,why is this?

Well, i believe its based on activity of your forum.

http://dollchitchat.forumotion.co.uk/

 So the more activity the more it's going to be?
Will there be a point when it wont keep going up?
This is going to be expensive,even with donations!

I am not sure if it stops. Also, the concerned person who can answer is a little busy at the moment.

http://dollchitchat.forumotion.co.uk/
Thanks for your help,i did notice it fluctuate between low and high at different times of the day which as you say would indicate forum activity. 

I did not mean based on the activity, as in the members who are online. By activity i meant the following.
1) Number of users registered
2) Number of visits you get everyday
3) Number of topics/posts made

Is this solved?