Lower revving than normal, especially in lower gears

Hello,

I'm driving a 1.2 Clio and in the last day or so it has started revving lower than normal. Normally I would sit in 5th at 30-40mph quite easily, but yesterday it started getting a bit shakey around here and had to drop it to fourth.

Today I'm finding that I am having to use a lower gear for a lot longer than normal while I'm building speed in order to build up the revs.

It doesn't notice as much when I'm in 5th on the motorway but I suppose it wouldn't at that speed.

Does anyone know why it might be revving lower?

Thanks in advance!