I can see why you're anxious, but at the moment there isn't any definite 'no', is there? The blood clot MAY not be heart-related - I have a friend who right now has a blood clot because a car door was caught in the wind and banged shut on her leg, then the blood clot travelled from the original injury. In your place, I would be asking the doctors to explain all the possibilities very thoroughly. Then, "not AUTOMATICALLY covered." What does that mean? When you have talked to the doctors so you know what they think is wrong with your mother, and why, then you will be in a better position to talk to the insurance company.