My Quebec tax return is showing a much lower taxable income than my federal return

Having a difference in the amount of tax owing/refund due between federal & Quebec is quite common. This is due to a number of factors but, usually, when the provincial amount is much lower, it's because you're eligible for Quebec-only credits - there are many.

I am very concerned about this! I am self-employed. Right now, my Quebec return says I have about half as much taxable income as my federal return. In the past, when a CRA did my taxes, they were always about even.

If you're concerned that you've forgotten/missed something, you can always do a comparison. Print a PDF copy of your current return and compare the lines to last year's return.