email problem

Yesterday, 4 emails were sent from my email address without my knowledge. 2 were sent to old contacts of mine whose email addresses are no longer active. 2 were sent to contacts currently in my address book. Each email had no message, just a link to click on by the recipient to a website link. I didn't click on any of these for fear of who knows what. The 2 recipients responses to me made no sense , so I don't know what the subject matter was. Any thoughts on what I should do or what was going on??