Need help for my mum

Hello people Im on here to get advice regarding my mum. Last year I guess she suffered a mental health breakdown it has happened before but its longer this time. She was on sertraline 50mg and things were good. Last year though long story short you can look in the infectious mononucleosis forum at my posts to see what happened to me. She went downhill and shes trying to get back to her old self. She is complaining of a weird depression where she gets headaches at night after trying to do activities in the day such as voluntary work. She is now on mirtazapine 45mg but she can't really sleep on it she was on 30mg previously and found she could have a good nights sleep on that. What could the headaches be? She says she feels limited because if she does too much the headache appears. Im wondering whether she should try and go back on sertraline as up until the breakdown she was good on that. Any advice on anything in this post ill be most grateful. The main thing is the headache depression though