Pulmonary Embolism Support Group
By far the most common form of pulmonary embolism is a thromboembolism, which occurs when a blood clot, generally a venous thrombus, becomes dislodged from its site of formation and embolizes to the arterial blood supply of one of the lungs. Symptoms may include difficulty breathing, pain during breathing, and more rarely circulatory instability and death.
Sometimes I think our lungs go through a healing process where they are "re-organizing" themselves, and that's when we notice things. (That's not a scientific opinion - just my general opinion as a way of trying to explain things to myself!)
Also, if you started working longer hours more recently, that may be the cause of it, too.
But like veeachjay says, you might want to run it by your doctor, anyway, since this has come up and it's different than what you've been experiencing.
Take care.