Gmail security?

Hi all,

I had an interesting situation where for three in 2 days I got a message
from gmail saying that I was locked out of my account. I was using chrome
to access it at home and Firefox 3 from work. Yesterday I saw a message
saying that I have purchased a piece of computer equipment from an online
computer store which I didn't. I rang the store and computer equipment
was picked up by someone over 100km from me. So obviously someone has gain
access to my gmail. I thought I had a secure password. Any idea?