As I understand it, her power level never actually changed. She's an energyvore, kind of - she sucks in power and pumps it back out as flight and strength and energy blasts. We've seen her absorb explosions and whatnot for a temporary power boost. What made her Binary form so badass was that she had a connection to a white dwarf star or something equally bottomlessly energetic like that; when she lost the Binary identity it was because that connection was severed, so she no longer had the juice to keep doing the really cosmic high-end stuff. She still theoretically could, though, if you charged her up enough beforehand.
What happened to her here could probably be explained as faulty energy regulation, or something happening to reconnect her to a high-end power source, or various similar theories.