There was an email sent to my contacts but not from me. How is this possible?

Regarding your question - There was an email sent to my contacts but not from me, I think you are looking at it in the wrong way. One of the following scenarios is probably happened:

The attacker gained your contact list and used their email to spam your contact list - In this case I suggest you look over apps/programs that are installed on the device you use for checking your email. Attacker has hacked your email account and stole your contacts and then spammed them using their own email so your email provider won't block them (They might connect through an API or try sending all kinds of spam that your email provider might have blocked) - In this case I suggest you change your password and keep alert for any suspicious activity (You might want to check how they managed to hack you) I also suggest you use haveibeenpwned this website to see if you were hacked.