Not possible to do accurately now I'm afraid on the past tests, but there are a couple of things that might help assess normal usage from now on, so you can maybe work back.
I assume you mean the initial speed tests were
this one? If so, before you start it going click the "Detailed output" box. That will then show you how much it is downloading and uploading. (It does short ones first that are always the same size).
You will get to see the approximate amount it uses for various speeds, though when the tester is heavily loaded it sometimes reduces the amount. So for working back it will be a bit approximate, for the future you would need to note down the amounts before they disappear when the results are loaded.
If you have registered on the Main Site, which is a separate login but you can use the same username and password, then the speed test results are stored in the Connection profile you can set up. Unfortunately it doesn't store the amounts.