Period Performance

Hi ive been asked to put in some costs for one of our projects.

the month of november has costs of £18.31 so i status the activity and update to next data date. after data date i store period performance so £18.31 shows on resource assignement tab and activity usage spreadsheet etc.

Im fine up to here.

Now when i try and put in the next months costs of £46,546 and store the period the two figure spread together.

i want the previous saved cost to freeze and not change but every time i try to change the next months period/cost they spread.

what seems to be the problem?