What are the reasons for constant light nausea (no vomitting)? I usually don't feel it while in movement and don't feel any kind of pain.

Many possibilities. Many possibilities - could be gastrointestinal, nervousness, brain related, inner ear related, medications, diet related, elevated CSF pressure, elevated intra-ocular pressure, endocrine / hormone related....Etc. Very broad topic. Need someone to ask appropriate questions like associated symptoms, timing etc. And check many tests.