As a former EMT and a mom of 3 crazy kids (didn't read the other responses), watch her for the following: uneven or completely dilated pupils, dizziness, vomiting, crossed eyes, uneven balance, becoming very sleepy (even if it's not naptime), sudden complete loss of appetite, gibberish (not the normal 2 year old kind), or sudden laziness.
You know your child better than anyone, you know if they're acting normal or not. Trust your gut, use your discretion, keep some ice on that bump! Even if there had been tons of blood, it's not an immediate reason to rush to the ER since the head bleeds like you wouldn't believe!!
If she's already playing, she's probably fine, so relax :) Just watch her for the things I mentioned above. Kids give parents heart attacks with this stuff, but are generally incredibly resilient and are fine!!