Depending on how you send e-mail, your current IP (at that time) might be present in the
Received: header of mails other people got from you.
Try and send an e-mail to yourself and look at its source code (how to do that depends on how you access your mail) and compare the addresses that show up in the various
Received: headers to see whether your current address appears there.
If it does, you “just” have to somehow get one of the e-mails you sent at the time you’re interested in. You might have sent one to a friend or to a mailing list.
(Yes, the chances to find the IP this way are pretty low, but depending on how important it is to find it out it might be worth the effort.)