Since the injury is already two days old and there is no accompanying symptoms of intra abdominal injury
like persistent vomiting
,hard stomach,any urinary or bowel complaint,it rules out any internal injury.
The hard knot is due to swelling and some bleeding in muscles of abdomen at the site of injury.The local bruise
Just local hot fomentation and pain killers like MOTRIN
should be enough for you.
There is nothing to worry.