Working hours


My boyfriend was sectioned less than 5 months ago. I changed from full time to part time hours. Although he’s out of hospital now I still ensure he is mentally able to go into work, motivating him to do so when experiencing terrible anxiety. I also am on top of his meds and secondary mental health care.
This morning my boss sent me a strict email saying that my partner is better, that me going part time has been to the ‘detriment’ of the business and has demanded me to go full time on Monday, giving me only a weeks notice.
I went part time to take pressure off of myself and to look after my own mental health as I have been through a demanding and traumatic time.

Is this actually allowed?!