Vauxhall Corsa engine revs when chaging gear but my foot isn't on the accelerator

This only started happening on Friday, on my way to Liverpool, when i am changing up a gear and i have pressed the clutch and released the accelerator, the car is revving quite badly, like im really pressing down but my foot is fully off the pedal. It is worse when i stop and have to set off again, like at traffic lights or a junction but its revs worse (i don't know the technical terms but it reaches up to no 3 on the revs)