Thanks for your query
As you are a patient of Marfan syndrome
and this is your symptoms we the doctors should have to exclude aortic aneurysm
first. So please have an repeat MRI to assess your aorta recently. Because in this clinical context aortic aneurysm is the strongest option we had to explain your symptoms. Along with this a need to have an ECG in order to exclude any rhythm abnormality of your heart and an Echocardiography
to exclude valvular heart disease
specially to exclude aortic regurgitation
. Also fix an appointment with cardiologist and get yourself locally examined. I think by all these you can have a diagnosis.