my email acct hacked?

ok today i checked my email and i see i got an email from my own email address with some sort off phising scam. i did a reply to it and it was from myself so how is this possible , ive changed my password etc 2 weeks ago also scanned my pute fully with avast but theres nothing on my machine(btw i have the the full avast not a trial), so how is this possible?

scan your comp with MBAM and attach results here

and change email pass again, just in case(from other computer with no potential malware, if you can)

ok did the full scan

Malwarebytes’ Anti-Malware 1.46
www.malwarebytes.org

Database version: 4514

Windows 6.1.7600
Internet Explorer 8.0.7600.16385

1/09/2010 3:47:52 AM
mbam-log-2010-09-01 (03-47-52).txt

Scan type: Full scan (C:|F:|)
Objects scanned: 192407
Time elapsed: 23 minute(s), 1 second(s)

Memory Processes Infected: 0
Memory Modules Infected: 0
Registry Keys Infected: 0
Registry Values Infected: 0
Registry Data Items Infected: 0
Folders Infected: 0
Files Infected: 0

Memory Processes Infected:
(No malicious items detected)

Memory Modules Infected:
(No malicious items detected)

Registry Keys Infected:
(No malicious items detected)

Registry Values Infected:
(No malicious items detected)

Registry Data Items Infected:
(No malicious items detected)

Folders Infected:
(No malicious items detected)

Files Infected:
(No malicious items detected)

It is actually possible to make a fake sender e-mail. If you register on weird sites… they might spam even in this way.