The ER is for immediate problems that need medical attention to resolve immediately. Since your daughter's immediate problem (the pain and the faintness) resolved while she was waiting for a nurse, there really was nothing more that the ER could do (since they're not in the testing/diagnosis business; they're in the emergency treatment business).
Your daughter needs a full top to bottom physical and an EEG, CBCs, thyroid panel, iron and glucose testing, the whole shebang. Pain following a fainting spell doesn't sound at all like any epilepsy I've ever heard of. I have a relative with the type that causes grand mal seizures, and he doesn't even feel pain when he "comes to." He just feels physically exhausted, and sometimes it leaves him with a migraine headache.
He knows when he's about to have a seizure; he feels a little woozy and unstable. Since he knows what that feeling means for him, he knows to yell out for someone in the house to come to him, then he'll take out his bite-stick, and lay down. He usually has around 30 seconds to do so. (A bite-stick in our case is a flat wooden tongue depressor with around half an inch thickness of tape wrapped around one end. It gets placed between his upper and lower front teeth, to prevent him from biting his tongue off when he goes into a seizure. The tape provides padding so he doesn't chip a tooth or splinter the wood in the process).