My eyes have felt a little bit gritty over the past few days. no other symptoms like swollen or red eyes. how can i fix this? what causes this?

Artificial tears: If you are on synthroid, (thyroxine) you might have graves disease, a component of which is known as thyroid orbitopathy. See an ophthalmologist/oculoplastics specialist. In the meantime, lubrication of the conjunctiva with over-the-counter artificial tears should reduce the grittiness. Dry eye syndrome is common, but not very common in a 20 year old. See sjogrens syndrome and keratitis sicca.

Dry eyes: Your symptoms sound typical of some dry eyes. Try using some artificial tears several times a day for awhile to see if this helps. I notice that you're on thyroid placement therapy. Also make sure your doctor checks your thyroid levels if that has not been done for awhile.