When I checked my email today I suddenly found an email from clients@000wehost.com saying that “Today is exactly 2 weeks since your website https://sequined-house.000webhostapp.com is online.” I’m totally confused. I have never registered such website.
You may have registered on one of 000webhost partner pages.
No. It doesn’t sound familiar at all. (sequined-house)
Just logged in to my control panel, and I can’t even find that website! Surely it’s not someone hacking into my account.
There is no problem with 000webhost mailing system. You have created account on 5jelly.info
Well.
I did explore the 3jelly and 5jelly site, but 
I really have no idea about what this “sequined-house” website is.
By the way is 5jelly 000webhost’s? Why do you know? 
is auto-generated website name, which is mandarory, but you can attach your own domain later.