the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
Do not worry. What you did is not the oath of zihâr.
The oath of zihâr is where a man swears an oath declaring his wife to be “like the back of his mother” in order to make sexual relations with her unlawful to him while not actually divorcing her. This was a common form of oppression that was perpetrated against women in pre-Islamic Arabia.
If a man does this sinful act, he may not have sexual relations with his wife again until after he frees a slave. If he is unable to do this (which is the case today in the absence of slaves) then he must fast for two consecutive months before he can have sexual relations with his wife again. If he is truly and absolutely incapable of fasting, then and only then may he turn to the next expiation, which is the feeding of 60 poor people. [Refer to Sûrah al-Mujâdilah: 3-5]
If you merely called your wife your mother, meaning that she is behaving like your mother or nagging you like a mother would nag someone, then this is something else entirely.
Source: Islam Today