Hi Cheeryface
I don't know quite where to start because I really want to say don't worry it's most likely nothing but I also want to say please go back to your GP and ask him/her to refer you to the neuro department at your local hospital if you are the least bit unhappy. I'm sorry for the long post but this is an issue I'm absolutely passionate about.
My DS had a brain tumour when he was 8 which was initially mis-diagnosed as being an unhappy child or migraine by the consultant. I knew he had a brain tumour as did our GP - text book symptoms - but until we saw a med student at a later clinic at the hospital nobody took us seriously even though I did all I could. She has now specialised in paediatric oncology mainly because of my son and I can never stop thanking her whenever I see her 14 years on.
The problem is that brain tumours which ARE rare have symptoms which are similar to many, many non serious illnesses and conditions - migraine, trapped nerves, hormonal imbalances, blood pressure problems, eye conditions etc, etc.
Cheeryface, it is very, very, very unlikely - incredibly, extremely unlikely - that you do have a brain tumour but if you are still worried you MUST, MUST go back to your GP and explain that you feel dreadful and describe all your symptoms. Ask to be referred if necessary. You might also want to consider keeping a diary of your symptoms so you can show your GP when your symptoms are at their worst/best. This might also show you that there is a particular pattern to your symptoms, maybe linked to your periods. I get hormonal migraines every month. Nasty but regular!
Don't panic or start researching brain tumours on google - you can convince yourself you've got anything if you do that. I repeat BRAIN TUMOURS ARE RARE but the important thing is to be satisfied that your GP is taking you seriously. Any GP worth their salt will happily explain to you why they think you haven't got one!
I hope I'm not accused of being a panic monger. That is certainly not my intention but until you've been in the position of dealing with a life threatening illness where diagnosis and treatment has been delayed - by 5 months! - because your concerns have been dismissed time and again then I don't think you can really comment. I just don't want it to happen to anybody else, but it still does.
Btw - my DS is 21, completely healthy and is now considered cured.
Cheeryface, you'll be fine. I have no doubt that there is a much more mundane and straightforward diagnosis but if in doubt don't be fobbed off and ask for a second opinion. Pay for it if necessary!