Salam alaykum
What comes first - obeying Mom or obeying Allah?
You can find from the Quran and from hadiths many examples about being disobeyed to parents like:
Allah says in the Holy Quran Chapter 17 Surah Bani Israel verses 23-24:
Your Lord has enjoined the following: You should not worship anyone but Him Alone! Treat your parents with great kindness; if either or both of them attain old age, do not even say uff to them; nor rebuke them; but speak to them kind words. Treat them with humility and tenderness and pray, O our Lord, be merciful to them, just as they brought me up with kindness and affection in my childhood.
Allah says in the Holy Quran Chapter 31 Surah Luqman verse 14:
14 And We have enjoined on man (to be good) to his parents: in travail upon travail did his mother bear him and in years twain was his weaning: (hear the command) "Show gratitude to Me and to thy parents: to Me is (thy final) Goal.
Allah says in the Holy Quran Chapter 4 Surah Nisaa verse 36:
36 Serve Allah and join not any partners with Him: and do good to parents kinsfolk orphans those in need neighbors who are near neighbors who are strangers the companion by your side the way-farer (ye meet) and what your right hands possess: for Allah loveth not the arrogant the vainglorious;
Al-Tirmidhi Hadith 4941 Narrated by Abu Umamah
A man said, "O Messenger of Allah (saws), what rights can parents demand from their children?" He (saws) replied, "They are (or your relation with them will determine) your Paradise or your Hell."
Sunan of Abu-Dawood Hadith 5120 Narrated by Mu'awiyah ibn Hayadah
I asked: O Messenger of Allah(saws)! To whom should I show kindness most? He (saws) replied: Your mother. I asked (again): Who next?. He (saws) replied: Your mother!. I asked: Who next? He (saws) replied: Your mother! I asked again: Who next? and he (saws) replied: Your father, and then your relatives in order of relationship.
Al-Tirmidhi Hadith 4939 Narrated by Mu'awiyah ibn Jahimah
Jahimah came to the Prophet (saws) and said, "Messenger of Allah (saws), I desire to go on a military expedition and I have come to consult you. He (saws) asked him if he had a mother, and when he replied that he had, he (saws) said, "Stay with her, for Paradise is at her feet."
Sahih Al-Bukhari Hadith 8.18 Narrated by Aisha
The Prophet (saws) said: "The word 'Ar-Rahm' (womb) derives its name from 'Ar-Rahman' (i.e. Allah). So whosoever keeps good relations with it (womb i.e. kith and kin), Allah will keep good relations with him, and whosoever will sever it (i.e. severs his bonds of kith and kin) Allah too will sever His relations with him!"
Sahih Al-Bukhari Hadith 3.821 Narrated by Anas
The Prophet (saws) was asked about the Kabair (the absolute gravest sins in the Sight of Allah). He (saws) replied: "They are:--
To join others in worship with Allah,
To be undutiful to one's parents,
To kill a person (which Allah has forbidden to kill) (i.e. to commit the crime of murder),
And to give a false witness."
Sahih Al-Bukhari Hadith 8.290 Narrated by Abu Bakra
Allah's Messenger (saws) said, "Shall I inform you of the biggest of the Great Sins?" They said, "Yes, O Allah's Messenger (saws)!" He (saws) said, "To join partners in worship with Allah, and to be undutiful to one's parents."
Al-Tirmidhi Hadith 3655 Narrated by Abdullah ibn Umar
Allah's Messenger (saws) said, "There are three people to whom Allah has forbidden Paradise: one who is addicted to wine, an undutiful son, and a cuckold who agrees to his womenfolk's adultery."
But:
Al-Tirmidhi Hadith 3696 Narrated by An-Nawwas ibn Sam'an
Allah's Messenger (saws) said, "A creature is not to be obeyed when it involves disobedience to the Creator."
This your story sounds very familiar to me - when I started to use hijab, my Mom shamed it so much that it took almost 10 years when she refuse to walk with me on the streets but then she used to accept it. Difference is, I was at that time older when I became Muslim and choosed to wear hijab (I was almost 30).
The only advice I can give to you is that try to explain (kindly) to your Mom you use hijab because you fear to disobey Allah. I make dua that your Mom will understand it and will allow you to use it.