From what i understand you want to check the previous month ?
If you want to get the result you have to do the following...
For me this could be done while loading the cube... you can use FM:
http://www.se80.co.uk/sapreports/r/rsdr/rsdri_infoprov_read_demo.htm
Use it into the transformation to calculate your stock, with it you can check the previous month and months before also you'll need some decent abap knowledge do it. Also you can try to add it into your report basically it reads the data from the infocube based on the criteria you have given to it.
BR,Todor