toileting problems

My son will be  5 years old in 2 months.  He has been "potty trained" since he was 3.  Recently he has begun to urinate and defecate in inappropriate places.  Once it was the cat's litter box. I found him tonight about to urinate on the carpet in the corner.  I caught him before he did it, but there was a stain there indicating he has gone there before.  The other day at the playground he defecated in the play equipment.  His excuses have been that "he had to go real bad".  None of the times he has done this has he said anything about having to use the bathroom.  
Once he urinated on the carpet in my bedroom not 4 feet from the toilet, and he said "he had to go real bad".
I have talked with him about it, explaining that big boys use the toilet. I have punished him, I make sure he goes before we leave the house for anything, and usually remind him to go throughout the day as well.
He is a well adjusted boy in every other way.
I have tried to think about what may be causing this behavior...I don't believe it's a medical issue, because it's not just urine, but as I said, he has defecated in inappropriate places as well...
Is this normal? Is he just lazy? Is he acting out? How am I supposed to handle this?  Each time, I think to myself, "there's no way he'll do that again." , Obviously this isn't going as I thought.  
Can you help?