Why has my cat suddenly started pooping everywhere?

We have a 13 year old cat named Smokey with no previous medical problems except for the occasional bouts of pink eye and gingivitis. She is our oldest cat; she had a brother who died 6 years ago and before the brother died we had added a new kitten to our family which they both got along with, and about 3 years ago we got another kitten and they all get along just fine. However during this past summer she started pooping all over the house. We have 3 litter boxes and we keep them very clean but she still decides to Poopin our brand new carpets. My cousin is a bet and checked her out and said everything is fine but there must be a reason behind this, help!

Have you seen her do it?  Does she look like she's straining, or anything?

Is the poop a normal size and consitency?

Is this cat overweight?

Are all three litter boxes the same, and located in the same place?

These are just a few questions that occurred to me.

What changed in your house when it started?  Could she be stressed?  My cat used to poop inappropriately when she was stressed.