Just block them back. I had a roommate with an overbearing mother that made a throwaway account to follow my Insta. I ended up setting my account to private and blocking her account because we’re grown ass adults you don’t need to be checking up on your daughter’s friends to see if they’re good influences or whatever.
In your case they’re trying to appear unbothered by your existence but still wants to check up on you. They’re asking if you’re posting things about them because they’re definitely posting things about you and are projecting. Just block the account you suspect is there’s and don’t give it another thought.
Luckily instagram blocks other accounts they make but on tt I keep getting notifications of accounts following me and when I click on it it says the account doesn’t exist. And you’re right she posted targeted things and I unfollowed her to see if they were actually about me and I was blocked 24 hrs later. We were still living together too like why not just wait till after the semester it was very odd