In most cases, I'd go with the "Sent to wrong address" angle, but you've got a ready-made reason with the back issues. If your mom gets nosy, explain about the back. It can seriously be a hassle to try to move fast in the middle of the night with back issues, especially depending on how far away the toilet is.
But if she doesn't bring it up, there's really no need for you to. Just get the address fixed, complain about the lack of discrete packaging, and go on with your life.