Theotokos does not mean Mother of God. It means "God bearer."
The term came out of the Nestorian controversy. It was used to make the point that at no time after conception was Jesus only man, but always both God and Man. Nestorians believed that Jesus was man first, then became God. This heresy was repudiated at the Council of Ephesus, which used the term Theotokos or God bearer against the Nestorian term Anthrophoros or man bearer.
Thank you for the correction, I believe you're right. Obviously, not all Orthodox are up on their Greek!