There's likely more than one way to do this; but this is my way. Create a table for the monthly notes. The idea is that it would either have only 1 record for each month. [ Or, otherwise, a date field you could deal, and and a calculation to produce the month; read on.]
In your existing table create new field, type Calculation, named "cYearMonth" (however naming position you use), with the functions:
Year (Period) & Right ("0" & Month(Period); 2 )
The "result" will be "Number"; production, for today: 201503
Create a relationship from the tables, bases on the "_cYearMonth of either table.
Every record in your existing table (with a Period date field value) will match its "month notes". So the related value in "notes" would work in your "monthly report" [ it would not work so well in a Form view of multiple records however, unless you used a "date" relationship ].
P.S. The Right ("0" & Month(Period); 2) makes the result (number) work, as 20153 would not work so well.
You'd need a related table that all of your records represented by each "sub summary group" could link to. There, you can have one note per month. The Period field might work as a match field or you may need additional match fields to make this work...