Matrix

From publisher blurb:

She was a law enforcement officer forced into gladiatorial combat following the invasion of her homeworld. Eventually, she found herself a test subject in the invading aliens' experimental weapons program. Although the aliens believed their molecular disintegration device proved successful, in actuality it transported her a galaxy away. Finding herself on a strange new world, she also discovered the experience granted her control of her own molecular matrix allowing her to alter her density and size at will. Unable to return home, she decided to use her abilities to aid humanity as MATRIX.