Generally, refund is calculated when tax deducted at source is more than tax payable, if there is no tax deducted , you may be eligible for refundable tax credit such Working Income Tax Benefit, a benefit paid to low income taxpayers if income falls within certain limits, there are not many refundable tax credits now a days
In your situation, it can happen if your income was within threshold before entering expenses so you were eligible for credit and after entering expernses , you are out of that threshold, this is one of the possibilities to see detail of this credit , see link below
http://www.cra-arc.gc.ca/bnfts/wtb/menu-eng.html
