Contact Lens-Related Corneal Irritation: The patient's symptoms of eye irritation, clear drainage, sensitivity to light, and the sensation of something being inside her eye, coupled with her recent removal of the contact lens due to symptom onset, strongly suggest an issue related to contact lens use. The slit lamp exam findings of corneal irritation without evidence of a foreign body support this diagnosis, as improper use, poor hygiene, or an ill-fitting lens can cause such symptoms.

Dry Eye Syndrome: This condition can cause irritation, sensitivity to light, and a sensation of having something in the eye, especially in contact lens wearers. The absence of a foreign body on exam and the patient's symptoms could align with dry eye, particularly if the contact lens use has exacerbated the condition.
Allergic Conjunctivitis: Allergies can cause eye irritation, clear discharge, and sensitivity to light. Given the patient's age and the fact that she wears contact lenses, an allergic reaction to the lens material, cleaning solution, or another environmental allergen is possible.
Blepharitis: This is an inflammation of the eyelids that can cause irritation, redness, and a sensation of having something in the eye. It's common in contact lens wearers and could be a contributing factor to her symptoms.

Corneal Ulcer: Although less likely given the slit lamp exam findings, a corneal ulcer (infectious keratitis) is a serious condition that can present with similar symptoms, especially in contact lens wearers. It requires prompt treatment to prevent vision loss, making it a critical diagnosis not to miss.
Herpes Simplex Keratitis: This viral infection can cause eye pain, sensitivity to light, and blurred vision. It's essential to consider, especially if the patient has a history of cold sores or genital herpes, as it can lead to serious complications if not treated properly.

Acanthamoeba Keratitis: A rare but serious infection of the cornea caused by a parasite found in water. Contact lens wearers who expose their lenses to water are at higher risk. Symptoms can include severe eye pain, redness, and sensitivity to light, which might initially seem similar to the patient's presentation.
Fungal Keratitis: Another rare infection that can occur in contact lens wearers, especially if they have exposed their lenses to contaminated water or have poor lens hygiene. It can cause severe inflammation and vision loss if not treated promptly.