If a blood clot cannot dissolve naturally, it may make you feel dizzy. Some people faint since their body cannot fight the problem on its own. It is very dangerous if the clot detaches and starts travelling towards other organs, such as the lungs or heart. It makes breathing very difficult. Dizziness occurs when the person stands up suddenly after lying down or sitting. The bad thing that most people do not pay attention to this symptom thinking that this is something natural. Fainting is also a serious symptom that should not be ignored. Ask your doctor to observe you. As a rule, people who lose consciousness get trauma when they fall and hit their head or any other sensitive body part. Medical attention is required if you know that you sometimes pass out.