Nightmares when falling asleep on back. Its where I know im asleep and try and shout out but can't wake myself, why does this happen when on my back?

Sleep apnea? I wonder if you might have sleep apnea. Many people begin to lose oxygen when they fall asleep on their backs because, as they begin to relax, their tongue falls backward and closes off their airway, leading to suffocation. If your nightmares are about suffocating or drowning i would consider that this might be happening to you. Many people need too sleep on their sides just for this reason.
Perhaps. Your vivid dreams are perhaps of little concern, but, in passing, consider a sleep disorder with hypnagogic hallucinations. The positional description might suggest sleep apnea. If you are starting to experience non-restful sleep, and have daytime drowsiness, it might be time to investigate further. Talk to your doctor about this.