Cellular data/Wi-Fi problems with iphone 5s?

So last month i switched from a Galaxy S2 to an iPhone 5S with T-Mobile (and before i say anything let me just say I've always used no more than 1.5GB of data) and our cycle ends on the 8th of every month. I use my wi-fi A LOT when i'm at home, and ever since i switched to the iphone my 2.5 GB high speed internet wastes not even within 3 weeks. I searched it up and people say the same thing, but i'm not exactly sure what the problem is. I didn't think it was the wi-fi's fault because it shouldn't be using any cellular data, but now i'm skeptical. It's only been 18 days since our new cycle started, so how is it possible that I've used all my data when I've been using the internet as little as possible outside of my house? I've been specifically saving up data because this happened last month too, even though i switched my phone the same month. Is my Wi-Fi being used as cellular data? And what is LTE? Is that what's using all my data?

Get you're *** to an apple store