Hi lauryn0001, sounds like you have some serious symptoms. Have you been to the doctor and described these symptoms? Your diastolic BP is low (< 60).
Becoming dizzy and lightheaded are symptoms of low BP. Sometimes this happens because the brain does not get enough blood flow, I have read. The shower may have effected your BP, and made it go even lower. Sounds like you almost passed out. As for the purple and and grey shapes, I can only guess and say that your retina didn't receive enough blood flow through the retinal artery, during this event. They don't sound like floaters to me. "Floaters" are visual disturbances that are usually benign, or harmless. I have never read of any colors associated with them, except variations of white and/or amber, but it could be. I think that floaters are caused by microscopic specs of material within the viscous fluid of the eye. I have had transient ischemic attacks (TIA), and a light stroke, but this does not sound at all like a TIA, or stroke. To me a TIA is like a light stroke, that doesn't cause any damage. The lack of blood flow to your brain, could have caused the feeling of "doom" you had, that made you want to sob. The event could also have immediately scared you to tears.

You are very young and not overweight. It would be very rare for you to have developed heart disease, by age 18. I don't want to scare you but some congenital defects of the heart, don't show up until the latter teen years. I know very little about congenital heart disease, but I am old enough to recall more than a few instances of where young, high school athletes have had their heart defects to become apparent, while participating in sports. If I were you, I would see a doctor soon for bloodwork, and make sure they check your thyroid function. Hope you get better soon