New user Help needed malware

Hello

I have a problems that I am unsure about. Two times in the last 6 weeks my Yahoo email account has sent mail without a heading to everyone in my Yahoo address book. There is nothing in my sent items folder to show this but I have replies from my contacts saying that they have from me mail with a link to some unsavoury  things.  I looked on yahoo and eventually found a list of my log in times and this was sent in the middle of the night from Indonesia and says Yahoo Mobile and as I live in the UK  it wasn't me.

So what I am asking is   Can this be something that is hidden in my computer or is it a Yahoo problem.

 

Regards Ken