Possible Causes for Constant Nausea?

Hello. So for the past few months I’ve been having constant nausea accompanied by headaches which never seems to go away. I drink plenty of water and I’m on some medicine, but I’ve been having this nausea before transitioning to this medicine. I feel fine in the morning but it seems to get worse as the day goes on. It’s a pain because I have emetophobia, the fear of throwing up. My dad’s girlfriend says it could be because I have a bad diet, but I’m not sure, and I don’t want to see a doctor about it yet because I don’t want it to turn out to be nothing and cost my parents money. Does anyone have any ideas of what it could be?