I've been having a lot of internal eye pain, what could be wrong?

Lots of things: Could be as simple as presbyopia with eyestrain or as complex as partial angle-closure glaucoma. Best to have this one checked out to rule-out serious problems with the eyes. Sometimes diseases such as optic neuritis present with "internal" eye pain.

Eye pain: Causes of eye pain include foreign body, eyelash poking the eye, infection, sinus pressure, iritis, scleritis, tumor, optic neuritis, choroidal hemorrhage, trauma. If your pain lasts more than an hour or two, call your local ophthalmologist, especially if there is any change in vision.