What you are experiencing sounds like a type of orthostatic pre-syncope. This is where standing from a lying or sitting position causes blood to fall from the head to the feet by gravity, depriving the brain of blood flow, and causing the feeling of dizziness
. Since your symptoms seem to be worse while in the shower, then I would venture to guess that your symptoms are related to a vaso-vagal reaction. This is where an external stimulus such as a hot shower, a sudden scare or startle, an extreme emotional state can cause a sudden temporary drop in blood pressure (example is when a person faints after seeing blood). Vaso-vagal reactions are not dangerous except for the risk of injuring oneself after a fall. Some patients with this issue benefit from a diet higher in salt (I know that sounds crazy).
I suggest that you schedule an appointment with a cardiologist
. He or she can take a more detailed history of your symptoms and perform a physical exam including your pulse and blood pressure while standing and sitting. The cardiologist may also want to get an ultrasound
of your heart. I think this visit will result in some advice which will help your dizziness.