How would I know if I had a pulmonary embolism? Would it occur suddenly or gradually? And if I have low blood pressure does that mean im in the clear?

Pulmonary embolism. Occurs suddenly, associated with shortness of breath, pain with deep breath, sometimes coughing up blood. Often caused by a clot in the leg traveling to the lung. This condition requires immediate treatment, preferably in the hospital.
No... Some patients with pulmonary embolism are completely without any symptoms so you could have one and not know it. If the clot is big enough, most patients will have sudden onset of shortness of breath, chest pain that worsens with breathing, etc. Having low blood pressure does not protect you from having a pulmonary embolus.
Usually sudden. Typically, symptoms of sudden chest pain and shortness of breath, with or without coughing, coughing blood, light headedness, swelling in the legs, fever or even syncope. Quite often however, symptoms can be there for a few days before you notice something is wrong. A low blood pressure is usually a bad sign if it is caused by pulmonary embolism.
Spiral CT. The best way to know if you have a pulmonary embolism or not is to have a spiral ct scan of the lungs with infusion.