What phone do you have? If you have an android phone you can check the data usage in Settings then Data Usage and it should display which applications are possibly using your data in the background. If you have access to wifi at home be sure to keep your phone connected so that you won't use it when you're home. Another thing to do is simply shut your data off so that it doesn't use it when you're out and about.
When it comes to your plan, you can always change your plan to a higher plan in order to get more data. Keep in mind that when you change your plan to a new one the old plan is no longer available for you to get back. Or if need be you can use more data in your plan, you'll just be charged for the overage rate of your data (for whichever plan you have). Here are Koodos current plans so you have an idea of what is available: