Is it only me or has it ever happened with you?

you are obviously a nice person and do things for others regulalrly. People take advantage of this and can take things too far. You are not idiotic, you are clear about what your housemate's intentions are. Just be firm and say no. Dont worry about it, nor feel bad. It is your property and you have the right to say no. Good luck and don't change - you seem like a nice person.