Blepharitis: This condition is characterized by inflammation of the eyelids, leading to redness, swelling, and a gritty sensation. It often affects one eye at a time and can cause discomfort without necessarily producing crusted eyelashes, especially in its early stages.

Conjunctivitis (Viral or Bacterial): Although you mentioned no crusted eyelashes, conjunctivitis can still be a consideration, especially if the gritty feeling is pronounced. Viral conjunctivitis might not have the same level of discharge as bacterial, which could fit the absence of crusting.
Dry Eye Syndrome: This condition can cause a gritty sensation and sometimes inflammation of the eyelids, although it typically affects both eyes. However, it's possible for it to start in one eye or be more pronounced in one eye.
Allergic Conjunctivitis: Seasonal or environmental allergies can cause one eye to become red, swollen, and feel gritty due to an allergic reaction.

Acute Angle-Closure Glaucoma: Although this condition typically presents with severe eye pain, blurred vision, and sometimes nausea and vomiting, it's crucial not to miss it due to its potential for permanent vision loss. The redness and swelling could be part of its presentation.
Orbital Cellulitis: This is an infection of the tissues surrounding the eye, which can cause swelling, redness, and pain. It's a serious condition that requires prompt medical attention to prevent complications.

Sjögren's Syndrome: An autoimmune disorder that can cause dry eyes and sometimes inflammation of the eyelids, among other symptoms. It's less common but could be a consideration if other diagnoses are ruled out.
Thyroid-Associated Orbitopathy: This condition, associated with thyroid disease, can cause swelling of the eyelids and tissues around the eye, leading to a gritty sensation and discomfort. It's less likely but should be considered in the differential diagnosis, especially if there are other symptoms of thyroid disease.