She discovered her math did magic.
It started out with harmless things, numbers glowing in the dark, sums humming upon solving. Things began to become more noticeable the day a particularly complex differential equation burned a hole straight through the paper and into the wood of the desk underneath, leaving a smouldering mark shaped like parentheses.
That day, she started to pay attention to the effects of her work (after hunting down a damp rag to extinguish the affected patch), taking careful notes and keeping a fire blanket at hand.
Soon she figured out which kinds of math were more harmonic, resulting in gentle light radiating from the ink on the page to keep her company. Other aspects went less well; especially certain types of fractions wreaked havoc more often than not, setting off tiny explosions either immediately or with a delay in direct inverse proportion to the denominator in case it was even and less than thirty-eight on a Tuesday.
After a few weeks she managed to create her first portal, tiny but still big enough to reach through the indices for the mug of coffee left in the kitchen two rooms away. Not much later, she travelled by the numbers and started writing into the sand of lonely beaches, trying to shift reality in subtle ways but not quite sure how far to go into touching the fabric of the world. Maybe just knowing she had the power to stop the universe was enough.