Historic bills keep changing and random huge Balance Adjustment has appeared with no explanation

Can anyone help answer why my bills keep changing and why I have a huge balance adjustment on my account suddenly?

I’ve been paying £75 direct debit for my two bed flat but received an email recently telling me it was going up to £452 a month. 

Shocked, I logged into my account to find all my bills from February were £300+ even though I could swear it was only recently I was in credit. 

I’ve checked back on my emails and as recently as May I was £95.62 in credit. 

I logged on to check my bills again today and now they’ve all changed again, dropping to around £150 a month but I’m still in a similar amount of debt because there’s a new Balance Adjustment for £975.58. 

There’s no details about what this balance adjustment is or a breakdown at all and no explanation for why all my bills are now different again. 

I phoned the support team who said it’s something to do with SSE but they couldn’t give a breakdown either and simply offered to reduce my direct debit by 15% which is still extortionate. 

I explained when the property moved to OVO I worked out with the support team then what we owed and already paid it in full. 

Just very confused and stresses as there doesn’t seem to be a way to resolve this, can anyone advise?