This can happen on low pressure open vented systems...it is only a problem if you have a mains pressure system as there is a non return valve as part of the valve gear...if operation has returned to normal then i would guess that there is no problem but if you have any doubt call your local plumber to come and have a look...or even just ring your local plumber and have a chat,and if he thinks it needs a visit then get him in. As a new cylinder costs around a grand+instalation whats a hundred dollars to get your local plumber to make sure things are ok!.