Cloudy urine from my 4yr old- help!?

For 3 weeks now, my four year old son has been excreting very cloudy urine. Towards the end of his urination he has a white 'cloudy' discharge which sometimes causes him some pain. It is not at each urination but maybe every other one. Because he is in plaster on both feet he is urinating into a bottle as a easier means and I'm able to observe this. Sometimes there is a white discharge in his pull up at night. We have been to the doctors last week with a specimen but when tested by dipstick showed absolutely nothing, and was even sent to a lab and again nothing in the sample. But this white stuff is still in subsequent urine samples! What could be causing this??? Anyone been thru this?