my bills are now coming monthly and I did not change it from quarterly. Can I change it back ?
No, BT do not offer quarterly billing any more, only monthly.
Did you recently make any account changes ? BT are not changing the frequency unless a change is made like renewing the broadband or taking new services.
It's a step forward in my view, if only because it means the death of the awful Monthly Payment Plan which sought to rationalise payments but caused more grief than it was worth.