It is common to get bruising on or near the site of a knock.
You would have noticed straight away when the incident happened whether your daughter suffered with concussion, because she would not have been her normal self, would have been confused, cold and clammy and her eyes would not have been able to focus on you.
If she is complaining of any head pain, feeling dizzy or feeling unwell, then you should get her seen by the doctor. The bruising will fade in time. The bruising would not have come out as bad had you applied a cold compress for about 5-10 minutes straight away after her bang.
Best wishes.
Does she seem okay otherwise, has she been sick unwell..If she had been concussed I suspect there would have been symptoms by now ..All the same it wouldnt hurt to ask for medical advice ., bruises can last a while ..good luck