How are you feeling? If ill, go to the walk-in centre. If not, some ice and rest should fix you right up! If the pain doesn't ease, take some painkillers, if they don't work, go to the walk-in Hope you feel better soon, and advice: don't hit your head, it hurts
Out of frustration I kept punching my head on the side and now I think it's bruised/swollen/lump.
There is a hard bump.
Will it go down?
It looks like the side of my head has slightly expanded.
Head injuries are a bit sensitive so it might be worth you getting it checked out (call GP surgery or 111 for advice on that). Otherwise, put a cool pack on it and it should sort itself out eventually. If you do get any feelings like dizziness, tiredness or any other odd symptoms please call 111 because you may have done more damage than you realize. You can look on the NHS website for more information by googling something like "head injury NHS". That should tell you what to watch out for and what to do about it.
Obviously in the long term you're going to want to stop punching your head. I understand it can be difficult if you're full of frustration, but when you're getting that feeling please try to find an alternative. Maybe punching a pillow or drumming on a desk. You could also try relaxation exercise like controlling your breathing, counting, thing of song lyrics etc. If you're getting times where you're so frustrated you hit yourself I think it would be worth you getting some support like counseling. You can talk to your GP about it and they can tell you what options there are.