Oh my goodness. Poor you! :-(
I too suffer with that side of things, I'm lucky if I go to the toilet once a week... but FOUR weeks?! Jeez, you must feel awful.
I went to see a doctor last week, who was really helpful and gave me some Movicol sachets. He told me to take them twice a day, but still no movement (and I drink loads and loads of water) so I've now upped them to six a day. I've seen a slight change, but not as much as I'd have expected with such an increase.
I am having bloods taken on Friday, although I'm not sure what it's likely to show up, so maybe request the same from your GP?
I did actually ask him about colonics, as I was also thinking of going down that route, but he seemed to think it wasn't a great idea. No idea why, he just said "I doubt you'll find any doctor who would recommend that". He also said "The sound horrific"! lol
I feel really bad for you, you must feel terrible. I know how crappy (wrong choice of word) I feel, but four weeks is such a long time to be... er... 'blocked up'.