Yahoo problem - spam or real problem

Hi all.
I need help with a possible mail account breach.
I have an email account with yahoo. When I logged today at email I saw 2 or 3 spam messages sent to several friends emails and including my own at 00:50 am (email was in my inbox). It contained a link to some trojan. I as soon as I could sent them a warning from another PC (clean one) not to open today's emails sent from my address.
Some of my friends that got that email reported to me that they clicked on that link and trojan tried to install on their PC but link was blocked by their AV. My wife also clicked on link (she didn't see my warning email) and my Avast AV blocked the connection.
I scanned my PC earlier this week with avast, malwarebyte and Spybot s&d and nothing was found, and I also scanned PC just before I wrote this and it's clean according to report. Installation of my Windows 7 OS is fairly new, I reformatted my PC less then a month ago. Everything on my PC works normally and there are no problems.
Problem is there isn't any sent mail to anyone today from my account, and since my PC seems clean, I suspect it wasn't even sent from my email. Maybe just someone used contacts from someone's (maybe one of my friends or collegues) infected PC and sent it via those online services that enable you to write any email address in "sender" field?
Is that a possibility? How can I check whether it's sent from my email account or not?
I logged today to my yahoo email account from work and changed my password.
Can you give me any suggestions on what to do and what possibly happened?

Thanks in advance, Natsuke.

 

Thanks for fast reply. I already changed my password so theres not much to do except switch to another mail account.
Thanks for help, just needed confirmation of what I already thought there happened.