Lower belly pains

Every month from the first time we try until the start of my period I have these sharp pains in my lower belly.

As soon as my period starts it stops.

It is mostly painful when I move around.

It only starts when we had our first go of the month and I have been having this for a few months in a row now.

I have gone to the GP and had a echo (and internal) investigation (sorry, cant find the correct word for it right now) and everything was spot on.

I was just wondering if anyone else has (had) this and if they know what it is.

I apriciate any help as my GP cant.