For the past few months i have felt constantly sick, anti sickness tablets don't work & i always feel "hungover" even when i haven't drank why is this?

Don't know what you: Mean by "sick" or what anti-sickness tablets are. If you are talking about feeling nauseated, both of the drugs you list, and in particular the codeine, can be associated with feeling nauseated and dizzy, and cyclizine may combat some, but not all, of those feelings. Both drugs are sedating and can cause a hangover feeling. You need to talk to your doctor/prescriber about your symptoms.