I just worked 8 hours with a person who lives with someone who is covid positive. we both had masks. am i at risk?

Covid exposure: It depends whether the other person is also tested positive and whether symptomatic or not, whether the exposure was indoors or outdoors. In any case yes you are at risk, just that the risk varies based on the above factors. If the other person is also positive then you should definitely consider quarantining and monitor for symptoms.

Time will tell...: I know you said you were masks, but how close were you working? Were you frequently within 6 feet? We don't know status of your coworker - they certainly are at higher risk with a household exposure, but your risk depends on whether the end up getting it. They really should not even be at work - they should be quarantined for 10-14 days. Not sure why this person is putting coworkers at risk...