Actually, no. Your logic falls apart right at the statement “the only reason right is right is because God said so.” God does not just make up whatever rules He wants. This is a misunderstanding of the nature of God.
God is perfect. His rules come not from some arbitrary choice, but are an expression of His perfect nature. God isn’t anything in particular; He just IS. St. Anselm defines God as “that than which nothing greater can be thought.” God isn’t just a perfect being. He is the definition of perfection. Kinda boggles the mind, doesn’t it?
Anyway–God, in His perfection, cannot sin. He cannot break His own rules. He cannot make a triangle have four sides, nor can he make 2+2=5. Therefore, when you sin you are not disagreeing with God’s opinion; you are violating an objective Truth that is as much a part of God as your mind is a part of you.
This is all very complex thinking material. I strongly second the above St. Thomas Aquinas reading recommendation. It explains things far better than I can.