Does my extremely low BP mean I have a weak heart?

Depends. Age prior history of heart disease needs to be queried. Is the low BP causing symptoms? Some highly trained athletes have low pressure with good to excellent heart function. Many pts of all ages are fully functional with low bp.
Depends. This must be sorted out with a good history and physical with an internist or cardiologist. Care must be made especially if the low blood is accompanied by symptoms like fainting or chest pain/palpitations.