Energy bills went from £1700 in credit to -£600

I'm with Scottish power and recently input a meter reading after paying my most recent bill. After this I checked my account a few days later to find myself £1700 in credit, which I found weird but more than happy to have my future bills handled for the foreseeable. A couple days after this I received an email informing me of a new bill of £600. Is this a total outlier? Or is this normal when inputting a meter reading after a bit of time?