So you can be on fixed tariffs (5p/kWh) or a variable tariff whereby it changes depending on current prices. So for instance right now gas wholesale prices are high so these would be passed onto customers on variable tariffs, up to the energy price cap.
Companies were encouraging people to go on a fixed tariff, as these are not covered by the price cap, hence the advice from Martin Lewis et al not to fix as then the price cap would cut in.
However, it’s not a fixed price like your broadband is, whereby you can use as much as you want for £30 a month, it’s metered so every kWh of gas costs, and there is also a daily standing charge.
I’d check your actual prices, it should say on your bill your kWh price, and your standing charge, and let us know, because you could also be on a really bad deal.
However, as PP said, your direct debit is probably too low, so you’re paying less than you’re using, so need to clear the debt and pay your current usage, hence the increased direct debit.
Does your bill say your current debt? If you can clear this in a one-er, then your direct debit will be lower going forwards, but probably still more than £79.
Finally, utility companies like to balance out your usage, so going into the winter you have a credit on the account, and by this time of year it’s down to zero (as the coldest months have passed) and you’ll start building up a credit over the summer when you’ll use a lot less gas. If you moved over in the autumn, you’ll never had had a chance to build up a credit for winter, but it may even out over the year.