Threads That Bind (Havoc Chronicles Book 1) by Brant Williams

At 16, Madison has accepted herself for who she is: smart and witty, but overweight with thick glasses and the social life of a Tibetan monk. Everything changes her junior year of high school when her eyesight inexplicably corrects itself, and she rapidly loses weight. But her new look comes with an unexpected price. Madison’s first kiss with the boy she has had a crush on for years triggers powers she can’t control, almost killing him.

She discovers she is a Berserker, chosen to guard the world from the Havocs, ancient creatures brought into our world by magic thousands of years ago. Only the Berserkers’ life-blood can bind – and free – the Havocs. One Havoc is free and wants Madison’s blood to free another. Madison must master her powers and bind the Havoc before it kills her.

And if that weren’t bad enough, it turns out she is the first female Berserker since, well – ever.