I noticed tonight that my four year old son keeps going back and forth to the bathroom, but then nothing comes out. He is completely potty trained, and usually never even wets the bed, but the last few nights, he has. Has anyone else had this prob?
Take him to the doctor urgently.
If he is able to manage to do a pee, get him to do it into a clean potty so that you can take the sample with you in a clean jar or urine tube (if you have one). It does sound very much like he has a bladder infection.
Have a look to see if he has a red and sore penis, or ask him if it hurts when he pees. This is another sign that he may have a urine infection or an infection in the penis called Balanitis.
Also make sure that he gets plenty of drinks and water. Not too much before bedtime so he does not wet the bed.