My current girlfriend who I recently became serious about, were talking for the past 10 months. She told me today that about 5 months ago she hooke up with her ex. She swears that it didnt mean anything and it was an accident but obviously I find that hard to believe. I care about here but obviously I'm not trying to let her get an inch. What should I do?

EDIT: I have decided you all are right, I wasn't being Beta enough to her, a cease transmission has commenced. Thanks for the responses!