Yup. we had the same with British Gas.
We had an issue with our smart meter, so it was replaced in June and reset to 0. Because I'd had CONSTANT issues with these thieves at British Gas, I'd paid off any balance so everything was reset to £0.
I've been paying £85pm for gas and £87pm for electricity in June and July.
Looking at the smart meter, in July (as of last week), I had used £13 in gas and £55 in electricity. They tried up to my bills to £135 for each, so £270 in total. When I queried it with them, they said they hadn't looked at my smart meter readings, they had simply added on 64% to the bill which would be in effect from 01.08 - TWO MONTHS BEFORE THE PRICE RISES.
I pointed out that if I don't turn my heating on till November at the earliest, based on the fact that I'm currently in credit of about £70 with them, if I paid their "suggested" amount, I would be nearly £450 in credit before winter even starts.
I've told them to keep it as is, and then I'll just make top up payments weekly/fornightly based on my smart meter usage.