Yes, this is very normal. What's happened is that your standard deduction and exemptions (people on your return times $4,050) have by themselves reduced your taxable income down to $0.
You're right. At this point, a million dollars of child care or donations, or a million dollar rental loss can't reduce your tax below zero. Luckily, Earned Income Credit skirts around this and behaves similarly to the withholding you paid at work.
Please feel free to post any additional details or questions in the comment section.
