Calculate difference from previous month

I have the following table that looks as such

I am using the following formula to count all orders

numOrders = COUNTROWS(AllOrders)

I then calculate the total retailed orders based on whether there is a retail date

Total Retailed = CALCULATE (
    [numOrders],
      FILTER (
        'AllOrders',
        AND (
            NOT ( ISBLANK ( 'AllOrders'[RetailDate] ) ),
            DATEVALUE(AllOrders[RetailDate]) <= MAX ( BasicCalendarUS[DateKey] )
        )
    ))

I then calculate the total retailed orders year to date by using this formula

Total Retailed  YTD = CALCULATE (
    [Total Retailed],
    FILTER (
        'AllOrders',
        YEAR( 'AllOrders'[RetailDate] ) = YEAR( MAX ( BasicCalendarUS[DateKey] ) )
    )
)

Now what I want is to have it do a month over month calculation to see the difference. I am having trouble picking up a formula that looks to the previous month and subtracts it from the current month to give me the number. 

Below is a screenshot of what I am looking for. ( I left out some months for simplicity)

Any help would be greatly appreciated 

Thanks!

Derek