Why is my iPhone making my internet slow?

Ok so the other day the weirdest thing happened to my internet, so i noticed that my internet speed was running ridiculously slow and like always i tried resetting the router which usually solves it, but it didn't. It turns out the problem was my moms iPhone, i know this because i tested this, i would run a speed test without her phone connected and i would get about 14down/1up and then i would run a speed test with it connected getting about.5down/.1up. My mom wasn't using any apps or anything that would use internet so something on my moms iphone was using the internet without us knowing and a lot of it. What could cause this to happen? Also as of the past couple days its starting to act normal again even with her phone connected. Our Wi-Fi is only hooked up to my iPhone, my sisters laptop and my moms iPhone. I even tried restoring the phone and although it didn't even have any apps installed or no browser open it would still use internet! And yes we have a password for the Wi-Fi. Please give me a good explantation for this, this has never happened to me before. I also noticed that when my moms phone wasn't connected the light on our modem would barely flash at all but as soon as i connected it the light would flash contstantly.

I'm not sure what you mean by connected, but if you connect an iPhone to a charger, it will start a cloud backup if you have that configured. This can substantially slow your Internet connection until the backup is done (several minutes, depends on the size of the backup and the speed of your connection).