Cellular Data issues? Iphone?

So i've not long got my iphone and my phine company were charging more than my bill because i was going over my data, which was not possible as i'm always connected to Wi-Fi. He told me to turn off notifications and other little tips that could use data. I reset my statistics, and turned my cellular completely off. I turned it on for less than 5 minutes today, i had a quick scrool through facebook and twitter then turned it back off. I checked the Cellular Data Usage (which can be found Settings>Cellular) and it said i used 24.9 MB?! I only have 1GB data on my tariff. Is this a problem with the phone or company? Or is this how it always is? I was advised i would not go over 1GB cnsidering i'm always in my iPad. My boyfriend never connects to Wi-Fi he uses 3G cellular on at all times and he doesn't go over his data limit. What can i do?

Just make sure to turn cellular off whenever you are in Wi-Fi, because it will use 3G if you don't. Also don't watch videos or load to many photos while on 3G, that will chew through it. My phone plan is 250mb a month and I never run out so you have heaps of data don't worry.