Look for a full-time job elsewhere.
Examine your attitude to your mum.
If she believes you've become more tetchy, and that it's only been for the last few months, it coincides with your meeting your BF.. which obviously, you're implying.
When we meet someone new, it does alter our dynamic with others.
We want to be 'with him', so we don't want to be 'with them'.
We also want to flee the nest.
You are of age, so maybe you should be looking to find a F/T job AND move out?
Take an objective step back and examine whether there might be some truth in her perception.
It's nobody's fault, strictly speaking - there have been Life-Changes....But you might want to just see whether there is an element of truth, and what you might have to take responsibility for.
All I know is, when I met my first real BF, I turned into the teen-b*tch from Hell....
My poor mum!