Shy Girl by Mia Ballard

Lonely, broke and depressed with a serious case of OCD, Gia finds herself at a crossroads when her financial troubles lead her to Nathan, a mysterious and affluent man she encounters on a sugar daddy website. Desperate for a solution, Gia is intrigued by Nathan's unconventional offer: in exchange for living as his devoted pet, all her debts will be erased. But the longer Gia is in captivity, the more animalistic she becomes.