iPhone 5s connection priority (Wi-Fi / 4G)

My iPhone 5s always (since I bought it 14 months ago) connected to my Wi-Fi at home in priority over the 3G or 4G connection. Recently (for about 2 weeks now) it gives preference to 4G for some reason and my mobile internet service limit (200 MB) goes over in a few minutes if I'm watching a video, for example. Why would this happen? I don't remember changing anything in the definitions/options, only thing I did was an update to a new version of iOS, but then it only started to happen about 3 weeks after I did that update, so it seems like it's not related with that. Can I do anything to stop this from happening? Is it possible that I'm getting cuts in my wifi service and while the phone does not get connection with the Wi-Fi it starts to get it from the 4G connection without asking or even letting me know?