If sinusitis is in the early stages, is it possible that it can cause nausea?

Not usually: Sinusitis can be a result of viral or bacterial infection. Some viral infections can also present with nausea, but although these usually have symptoms limited to the GI tract. Bacterial sinus infections can lead to a large amount of mucus post-nasal drip which conceivably could lead to some mild nausea if swallowed. Could also be related to dehydration just from being sick or medications...