When I die in nether my spawn point changes

When I die in nether I spawn in my world and die again instantly without doing anything. So it's dying for second time for no reason. Next,I don't spawn in my house where I set my spawn point but where I was spawned when I first created my seed. And another issue is when I enter nether portal I go to nether but when I enter nether portal from nether I am spawned completely somewhere else in my world with another portal created somewhere else. I don't return to my house where I have created nether portal. That another portal where I come from is made automatically in the world. I destroyed that portal but still it keeps forming on it's own.