I would not change the field names each month. I would either rename them to have very short names such as
CM
CM+1
CM+2
CM+3
ie, "current month plus 3"
The Report will just be a Regular report. If you do want the detail of each project, then it is a list of all project names and then say those 12 or 13 CM columns.
if you need to export to excel for the totals you would need to make a summary report type.
I also suggest making a field to totals up all 13 CM fields called [# Active Months], and then use that as a filter on the reports. ie [# Active Months] >0.
