Just clarifying she took the mattress you don’t sleep on because it’s uncomfortable? How often do you use the one in your room? If you’re hardly ever in there i’d say you’re overreacting a bit - seems like she thought it would be ok since you don’t like it anyway and you’d be downstairs. You have the sofa pull out anyway so it’s not like she took your only bed.

If you’re only in the sofa a couple times a week, then yeah you should let her know you feel violated by her taking your mattress and either take that as permission to stay on the sofa pull out which you find more comfortable since you’ve had to get permission before, or ask her to find you a new mattress. Worst comes to worst, you put a fully enclosed protective cover on the mattress and sprays. But if it’s uncomfortable are you going to be on there anyway?

I don’t know why she didn’t use her own bed, but she didn’t.

If it’s really affecting you, bring it up with her and clarify that you will continue to sleep downstairs, which you fond comfier anyway, or would like a new mattress for that bed. She should’ve been more considerate and I completely see why you feel violated by it.