My data usage on my Iphone 4s has skyrocketed in the last month, although I am connected to wifi 90% of my day.

Question

From August 8th to September 7th I used 1211mb of my 5GB plan on Telus. I recently moved to another city to go to University and suddenly my data has rose to 5050mb used in less than a month. I would chalk it up to moving to the new city, except for the fact that I spend most of my day either at home where I have constant wifi or at the University which has free wifi across the campus. The only time I'm really on 3g at all is the 30 minute bus ride to and from school where I will often stream podcasts off of the podcast app and ocasionally use the alienblue app to browse reddit.
When looking at the hour by hour rate on my data plan I can see that in the span of driving to school on the bus I would use on some days 60mb and on others over 200mb in an hour period. I am almost always just listening to podcasts directly off of the podcast app during this time and I can't imagine they would take up that much data.
Strangely enough the highest amount of data spent most commonly is from around 9am-10pm, which is when I am in class no using my phone at all and should be connected to wifi. Apps running in the background wouldn't take up this amount of data would they? Do I revert to 3g from wifi everytime I lock my phone?
I've contacted telus and the technical support had no idea how it could change so drastically especially because it doesn't seem to follow any set pattern, he thought the phone could have a virus which I found not very likely.
On some days it says I've used almost a GB of data in a 4 hour period!
Does anyone have any ideas of what could have caused this sudden jump in data usage? Does streaming take up that much? Would apps running in the background add up to almost a GB in several hours? I'm utterly confused, because I can see no logical way that I'm using as much as they are saying.
I've also downloaded an app to track the amount of data I use on wifi and on 3g which also shows me WHERE I've used what data, which should help me pinpoint where something is going on.
Any opionions or help would be greatly appreciated, if you have any other questions let me know and I can answer to the best of my abilities.
Thanks!

Answer 1

*Update*
I thought I had found the offending 'app' namely "Songpop."
I regularly use "Songpop" on my phone late at night via my wifi connection.
All enteries on my phonebill are at midnight (even though I have a data allowance it was excluded from allowance). I switched 3G off on Friday, played "Songpop" at midnight and at the exact time I got a "Server Error" message.
I posted a message on the "Songpop" support forum.At first they said this was a problem relating to my ISP.
I have spoken to my ISP and asked if there are connection drops at the time I have been billed (ie to establish if my wifi dropped and 3G kicked in without my knowledge) and have been advised that there aren't any connection drops at that time.
"Songpop" have now responded that "This is an issue to take up with Apple Support"  see: http://support.songpop.fm/songpop/topics/playing_songpop_on_phone_via_wifi_but_i ncurring_download_data_charges_for_3g_useage?utm_content=topic_link&utm_medium=e mail&utm_source=reply_notification
Hopefully someone at Apple can respond ....