1. You say "I never use the internet out of the house." But bear in mind that checking your email can consume cellular data when you are not on WiFi, or occasionally under other circumstances.
2. You say "I turned off my cellular data almost three weeks ago ..." So the message saying you used 90% of your data probably encompasses your monthly billing cycle, of which the three weeks is just a part.
Anyway, you can keep Settings > Cellular > Cellular Data = "Off" if you wish, and that should solve your issue.
Ok first of all I NEVER use wifi out of the house. I don't check my email or anything.( And i already know email uses internet im not stupid)I only bring it with me incase my moms car breaks down or something. I don't text people or call people im not some girl who texts all day with people in the same room)Only once in a while for an emergency. My cellular data is already turned off. Meaning three weeks ago i when to
Settings> general> cellulardata>off or what ever order it's in. It says when I go to settings and I look a cellular usage that I have not used any data (everything says zero) oh and now I used all my data for the month when I'm only on wifi and I rarely bring my phone out of the house. Not to school or friends houses or anything except if my mom wants me to go to a store or something. So yeah that didn't help at all
It sounds like you already have a very good understanding of what type of iPhone activity in a non-WiFi environment should create data charges, and that you have selected the right Settings to keep from using "Cellular Data".
If your Cellular Service provider is one of those companies that has an iPhone App that you can use to check on your charges / usage when ever you want, you should download and use that App.
My daughters are on my Cellular Family Plan (even though they are fully grown and have families of their own) and they have discovered it only one or two TV Shows or Movies to use up our Monthly Data Allowances provided by our plan ...so they typically only watch those items in a WiFi Network environment. A couple of times though they were amazed at how quickly they got the warning "Free" Text from our Service Provider saying they had used over 50% of their monthly allowance.
If you take your iPhone out of the WiFi network and have Settings>Cellular>Cellular Data "On", you don't have to actually view your email to have Data Usage ...that is if you have "Push" email checking. Also, seen threads in this Community Forum where iPhone Users were getting Data charges at night as their iPhones were charging. You could search and find some of those threads and their Solutions, in the event that is what you are experiencing.
It is possible, that your Service Provider may be able to help you discover the timing of the highest volume of data charges in your case. Then you could probably learn how to mitigate the issue and ensure you don't get surprised by future Data Use and Charges.
Good Luck toward learning the cause. Again, it sounds like you are very aware of effective Settings and of the features that could cause charges, but it will be well worth your time to see if your Service Provider has information that will give you an assist.