I am afraid there can be serious issues at times. Infective endocarditis
is a common complication in such operated kids and needs to be ruled out n priority. Such symptoms can also be there with reduced body fluid volume short of dehydration
Take her to ER again, show them this second opinion
and insist for tests including a BP check and X ray
if felt necessary. Not to feel bad if tests come normal.. we want them to come to normal. Do insist for a blood culture
and fresh colour doppler of heart. All this will keep a fullstop to dia gnosis of serious issues.