This can happen without hacking into your account. The fact that you are able to use your account indicates that your password still works. However, to be safe, you may wish to log into your online account and change the password for peace of mind. The problem you are experiencing is because someone is sending mail with your email address, which is something that is a common hack. Such people do this, often just because they enjoy upsetting the lives of others. You might try creating a filter to identify such messages, but I have no idea how to prevent completely, as this also sometimes happens to me.