Former rugby player Thom (27) is reported to have become "increasingly jealous" after Brook spent time catching up with her ex-boyfriend Danny Cipriani.
Last week, the pair decided to call time on their two-year relationship.
Kelly met Thom in 2010 shortly after finishing her relationship with England rugby player Danny. And apparently Kelly has never really moved on.
A source close to the model said: "She's never stopped loving Danny. Even though there was nothing going on, Thom hated the fact she was seeing him and Kelly refused to end their meetings."
Former Scottish international Thom has already moved out of their LA home. The couple have been through a lot together.
In May 2011 Kelly suffered a miscarriage with their child.