I would definitely mention it, I don't think period pain should be that severe. Uncomfortable yes, but not so you are in agony... Dunno what might cause it though? Maybe even see if they can give you a referral to a gynaecologist as they would probably have more idea than a GP. I think you should have it investigated though, you must dread AF's arrival!

ETA: if you think you'll have trouble describing it to them (I know I often get tongue tied at the dr's ) maybe write or print out what you wrote in your first post, then you can just read it out to them or hand it to them.