From the sounds of it, your Yahoo email address got hacked. That's a very common occurrence, and does not involve malware on your Mac.
Now, as for your bank account issue, that's more troubling. It could be coincidence, but I wouldn't want to make that assumption. If you use the same password for your email account and your bank account, it could have been hacked. Or the hackers could have reset your bank account password and intercepted the reset notice that was sent to your Yahoo email. Or perhaps you provided sensitive information on some bogus site that you visited after clicking a link in an email. In any case, you need to change not only the passwords for those two accounts, but ALL your accounts, anywhere. Anything linked to that Yahoo address could be compromised.
But, again, none of this is likely to involve malware. Although there is Mac malware, it's fairly rare and often highly targeted at specific groups. See my Mac Malware Guide for more information:
http://www.thesafemac.com/mmg