GMail: is really so secure?

About 15 days ago someone entered in my GMail account sending one spam email to all my contacts about an asian site of ecommerce. I apologize with all my contacts about the SPAM.

The very strange thing happened is that my password was quite strong: upper case letters, numbers and special character. So I exclude a brute-force attack in favor of a GMail leak.

By a quick look to the Internet I did'nt found any similar attack to other GMail users, so I could be one of first victims before Google patched the leak or the victim was expressely me. But in this case why don't change the password once in? And why don't delete messages to make greater damnages?

I don't know. However I'll continue to use GMail for my private mail since the service is really amazing and fits very well for my needs. I just hope my case was, and will be, pretty isolated.