Why did my housemate do this like a spoilt brat?

So our tenancy began about a year ago, there was 5 of us to move into a 5 bedroom house. Two of my housemates moved in first, but before anybody moved in we decided on who would have which room, there were 2 large rooms, 1 medium and 2 small. Me and a guy mark were gentleman and said the 2 girls could have the large rooms which they asked for, while we had the 2 small rooms at the back, the medium room went to our other housemate. One of the girls moved in first, she had the largest room in the house, but recently admitted that when she first moved in she went into my small room and found that the mattress was more comfortable so she swapped it with her uncomfortable one and never notified me, all year I hated the mattress, and just found out that not only was I gentleman enough to give her the biggest room in the house, she also decided to be selfish enough to go into my room and take my mattress and replace it with one she thought was uncomfortable, which it was. It's just the principle, so selfish and spoiled of her.