My other half discovered that some dodgy person/company had managed to send instagram messages advertising handbags to all of her followers from her account. She changed her password immediately, but what could have happened here? Is it the case that a “hacker” had access to her full instagram account, or would they have used some tool that allows posting of messages via some kind of proxy without requiring access to her actual account? There was no record of other logged in devices on the security page of her account.

Update: She’s just been through her junk email folder and found a “We’ve noticed a new login” email from instagram yesterday, so I presume that means they were fully in to the account then. How they got the password is anyone’s guess, but could be any of the suggestions below. Thanks all for the responses.