Thank you for your query.
I appreciate your concern with the symptoms that you facing. I suggest that you get your blood pressure checked up while sitting and while standing. There is always some change in the two readings but a significant change can cause the symptoms that you have described. In view of your heart murmur
it is all the more important that you get a cardiological work up done.
To prevent such light headed feeling and vision going blank I suggest that you get up slowly so that your body get some time to adjust to the change. If the blood pressure readings and cardiology work up are normal then this should give your some comfort.
I hope I have answered your query to your satisfaction.
Wishing you all the best.