Difference between a hacked account and somebody just spoofing your address?

i don't think my account has been hacked, but contacts email me stating that i'm spamming them. i tried looking at the source, and it seemed to me that the actual ip address was not yahoo mail servers. i even sent myself the spam (ie. sender and receiver same address). I changed password, which made no difference; which is why i thing someone is spoofing the address rather than totally hacked the account.