Before panicking about having a brain tumor, let's look at some other things that could be causing your symptoms.
When was the last time you had your eyes checked? Vision can get worse gradually, and the effects of migraines, headaches, dizziness, etc. can be caused by eyestrain.
You say your eating a healthy diet, but have you looked at possible food allergies? High amounts of MSG can trigger headaches/migraines, dizziness and nausea in some people. And MSG hides in all kinds of foods!
It could be that you're body is fighting an infection (sinusitis/ear infection) that the antibiotics didn't manage to kill outright and that's why you felt better for a few days and then felt worse again. More and more bacteria are becoming antibiotic resistant. It could also be a viral infection, in which case, antibiotics won't work and you just have to let it run it's course.
If you're feeling nauseaus after you eat, that sounds more like some kind of viral infection. Try eating only bland foods, easy to digest starches, like rice and noodles, and see if that helps with the nausea.
You have my sympathies! My mom is an RN, too, and worked in critical care- If I wasn't bleeding out the ears, in her opinion I wasn't sick!
I'd suggest talking to her again, making it clear that you're really worried about your symptoms and that you need her professional opinion, not her Mom opinion.