It's a sinus infection. Your sinuses are openings in the skull so what they probably saw was an inflammation of the sinuses. Coming from a hypochondriac, you really don't have anything to fear,sinus infections are reversible and can be treated with antibiotics. Relax, don't stress yourself out about it because you'll be fine. Luckily, whatever hospital you went to had the decency to tell you what your problem is before discharging you. Now thar you know, focus on getting treated for it. Take good care of yourself.
The CT scan showed that you do indeed have a sinus infection.
It also showed that there is a possible, but not clear, problem with another area. Imaging is not a perfect thing, so the radiologist is asking for further investigation.
CT testing is good for some things, and MRI is better for other - so you have to see a neurologist for follow up. I would make sure that the one you see is experienced in treating Chiari malformations (ask when you call, which sadly is not a guarantee, the person answering may just say yes and not really know).
There is actually a forum here devoted just to that disorder and this would be better there.