Suggest Treatment For Knee Pain After An Injury

My daughter was playing basketball this evening and pulled up to take a long outside shot; she came back down awkwardly and felt her knee pop out and go back in. She said she felt nauseous at first and then felt a bit better, so was able to drive herself home and walk up the stairs into our house. She says that it is hard right now for her to straighten her leg out (she can do so slowly now). We have the knee on ice but wanted some advice about what to do next; should the knee be x rayed and should we take her to the emergency room, or should we take her straight to an orthopedist tomorrow? Do you feel there is ligament damage or could the knee be hyperextended or perhaps a bruised ligament?