let me stop you right there...
You will only pay for the energy you use.
PP haven't increased your energy prices, the lower than expected reading can mean a couple of things but it's nothing to do with the price of your energy.
PP was expecting your reading to be lower so it's asking you to double check and make sure that the reading you entered is correct.
Look at the balance on your statement, if it's persistently going down then you are using more energy than the direct debit is set for (the amount of the DD is based on your original quote). If this is happening you're using more than you were quoted for and you may need to increase the DD.
If its staying positive and it's anywhere near a months payment after you've paid the DD, which is about where it should be then all is well, just ignore the message.
The formula used to work out whether to send the message isn't very clever and quite a few people have got the higher or lower than expected message when there is nothing amiss.