confronting roommate about cat pooping on bed

Question
Whenever I leave my home for vacation, my roommate watches after my cat. She feeds the cat and changes the litter, however, she also has a cat so I return the favor when she leaves on vacation. The problem here is that my roommate apparently locks my cat in my room when I leave town. She has done this the past two times I've left the cat alone with her.  I confronted her the first time it happened. I came home to poop in my shower. This was easy to clean and the problem subsided quickly. This past christmas I left home to visit family. When I came back my cat had actually pooped on my bed. And it had been setting for a few days. I've washed and  dried my sheets, but to no avail! The smell still lingers! How do I get the smell out of my sheets and how do I figure out why my roommate feels the need to leave my cat in my room for however long amount of time when I leave? I would normally leave my cat at the vet while I am gone, but this can be very expensive.  

Answer
If I had a choice, I would definitely opt for the poop in the shower...easier to clean.

Is the litterbox for your cat normally in your room/bathroom, or is it placed there whenever you leave town?  If so, your cat could be reacting to being locked in the room when you leave as, well, to give our cats human reactions, that you are abandoning her. But...I'm not meaning to do the guilt thing here. If your cat also normally has open access to the household except when you're away, being confined to the one room could be spooky to your cat, who may not like being alone.

For the bedding, some fluids, or pee, may have soaked into the mattress padding, which is why you may still have a smell despite the clean linen. If possible, remove all extra padding (of you have any), and spray spray spray with those spot remover sprays, soak the area, then use clean towels (I buy batches of clean 'shop' towels which are perfect for cleaning up messes) to blot up any liquid.  I try to continue this for a while. it might not remove all the smell, but it will ease up on it, and leave less of a temptation for your cat to re-use the spot again. I also use an extra clean towel, spritz it with a fabric refresher like Febreze (no, it hasn't poisoned my cats, and I go through bottles of the stuff), place that flat on the mattress, then place my fitted sheet over it.

Is it possible to travel with your cat, or to have another friend either take your cat in for a few days or come into the apartment and actually do the job of a petsitter (sorry, having a bit of a flashback here). Closing a cat in a room for hours at a time, just coming in with fresh food, water, and scooping out the litterbox...even at a vets, your cat would at least get some human interaction....sorry, flashback again. I can understand your roommate being busy and all, but if she can't or won't understand why you need her to not keep your cat cooped up all the time you're away, then maybe you might need to get a petsitter. Maybe the cat and roommate need some clearing of the air (no pun intended).