urgent help think a stranger has accessed my imessage through a mac

hello i have just recieved a notification on my phone saying

Phone Number Added To ''Natashas Macbook Pro''

''Natashas Macbook Pro'' is now using (says my mobile number) for imessage and facetime.

does this mean that this person is using my imessage on there macbook? i do not know this person and it is quite worrying is there anyway i can stop this person using my imessage and facetime? as i think they can see my messages i send a recieve?

thank you