Have you considered IBS? It can cause things such as headaches, gassiness, nausea, etc. Lyme disease is also increasingly prevalent this year and mimics a lot of your symptoms.
Have you hade any ventures where you may have been exposed to a tick? How long has it been going on? And have you had any recent infections that you needed antibiotics for? (Lyme disease made me sick for several years before my ultimate diagnoses.. 3 doctors later.)
If your family has a history of brain hemorrhages and strokes, I personally would be all for a scan. A visit back to the doctor who prescribed the Cocodamol would probably be a good idea as they will try to narrow things down now that it didn't work.
I also want to mention that your spleen is under your left side of your rib cage. It can become enlarged due to viral or bacterial infections, cancers, inflammatory diseases, etc.
I'm not a doctor or anything, but I don't want to see you suffer so wanted to give you my take on things (I have a LOT of medical issues myself.) Good luck to you I really hope this helps!