There are multiple things that can cause this. One possibility is your computer may be infected. Another possibility is a bot has access to your email account. Also, these emails may not actually be coming from your email account.
First, you should run scans on your computer with some antivirus software. Sometimes malware can take over email applications such as Outlook or Thunderbird to send out email to your contacts, which often includes your own email address. If you run scans from a few different programs, it should remove most traces of malware.
Also, you should change the password for your email account. If a bot has your email password, it can send out any number of emails to any number of people. Changing your password regularly is often considered a good practice.
If the third possibility listed is true, then there may not be much you can do to stop the messages from being sent. Sometimes, spam bots are designed to imitate other email addresses to conceal where the mail is actually coming from. You may be able to adjust your spam settings to filter out specific email addresses.