Here are some of the reasons you could spot, besides miscarriage or your period coming on: ovulation, hormonal changes, implantation, cervical irritation, rough sex, illness. Figuring it out depends on where you are in your cycle and how much blood was involved.
Even a really, really hard hit to the stomach isn't going to directly result in bleeding down there; if anything you'll have internal bleeding/bruising.
Without more information it's hard to say what might be going on. Did you bleed heavily/has it stopped? How many pads? When was your period due? Did you ever take a pregnancy test? Do you have any symptoms of miscarriage -- fever, nausea, tissue passing, cramps that make you double over?
The chances of getting hit in the stomach, or even a fall, in early pregnancy actually causing a miscarriage are extremely small, almost nonexistent. The fetus has lots of padding and is surrounded by liquid. Mama's body is a great shock absorber.
Most of the time if you are pregnant and have an early miscarriage it's because of a genetic defect, not an injury or something you did.
I doubt the hit had much to do with it. Being hit in the stomach is more likely to cause internal damage, not vaginal bleeding.
You could of been pregnant and the hit made you start miscarrying...i would go to the dr to make sure of whats going on...because either way pregnant or not being hit in the stomach than starting to bleed is never a good thing