hi

having troulbe when pooing, find it painful, so painful tha the pain goes down 2 my left knee

any help plz and what is causing this?

Have you injured or sprained you back recently? You are having problems with valsalva, which is forcing anything be it breath of in your case excreta, out of your body.

I found this article and copied it :-
The Claimant, a welder, testified that he injured his low back pulling 25 foot lengths of steel from its shelf or bin when he felt low back symptoms of pain there and in his left leg.  He apparently finished his shift in pain but upon experiencing disabling pain with valsalva excreta activity the next morning, he called in sick and reported the injury to Mr. Temple.  He was referred to an urgent care facility.

Claimant previously experienced neck and low back pain in August 1995, including treatment for symptoms extending into the left leg.  He treated for three months with a chiropractor.

It could also be sciatica :- http://www.healthscout.com/ency/68/172/main.html

No doubt you will wish to see your doctor.
Hope all goes well.

cheers m8 is that what could have caused it me sprained my back?

Yes, that could well be the cause.