The Seller Admin > Pay Your Affiliates function can only generate a commission report for the last completed calendar month. To see commissions earned in previous months, you'll need to use the Transaction Log in your Seller Admin. This normally shows only the past rolling month of sales, but you can pick any date rage, specific product, and/or specific affiliate and click View Log to see only the sales matching those criteria, then you can click Download Log to export that log view to a file on your computer that you can unzip and open in any spreadsheet program like Excel, iWork Numbers or (free) Gnumeric.
You could download a separate log file for each affiliate, but if you have a lot of them, you may just want to download a log file for all sales within your chosen date range, then when you open that in your spreadsheet program, you can sort rows according to the "Affiliate ID" column, which will make it easier to sum up commission earnings for each affiliate. Note there are two separate columns where affiliate shares may be reported: "Affiliate Share (common for txn)" and "Affiliate Share (per item)".