I think I may have a brain tumor? I have mild eye pain on one side.

See your physician. Mild eye pain can be from many different reasons some more significant than others. The wisest thing to do is to go to your primary physician or alternatively go to an ophthalmologist or optometrist to have your eyes examined. They will decide if further work-up is needed.
Eye pain. Eye pain is caused by so many things- including -dry eye, irritation from contact lens, other external irritation/trauma ( rubbing, eyelashes/hair, debris etc), infection like viral, bacterial infection; paracytes , inflammation -from allergent such as cosmetics, etc- it is unlikely due to brain tumor, fortunately. However, if that continues to bother you- go to see your pmd to have it checked.