Contact Lens-Related Irritation or Allergic Reaction: This is the most likely diagnosis given the context of wearing contacts and experiencing red, irritated eyes without a change in vision. The irritation could be due to an allergic reaction to the contact lens material, cleaning solution, or poor lens hygiene.

Dry Eye Syndrome: Prolonged wear of contact lenses can lead to dry eye syndrome, which causes irritation, redness, and sometimes a gritty sensation in the eyes.
Contact Lens-Induced Papillary Conjunctivitis (CLPC): This condition is characterized by the formation of small bumps on the underside of the eyelid, leading to irritation and redness, often associated with contact lens wear.
Blepharitis: Inflammation of the eyelids can cause redness, irritation, and discomfort, which might be exacerbated by or unrelated to contact lens wear.

Acute Angle-Closure Glaucoma: Although less likely, this is a medical emergency that can present with red, painful eyes, and blurred vision. However, the absence of vision change in this scenario makes it less likely but not impossible.
Corneal Ulcer: A serious infection of the cornea that can be caused by improper use of contact lenses. It presents with severe pain, redness, and blurred vision, but the latter is not mentioned in the scenario.
Uveitis: Inflammation of the uvea, which can be associated with severe eye pain, redness, and sensitivity to light. It's a condition that requires prompt medical attention.

Phlyctenular Keratoconjunctivitis: An inflammatory condition of the cornea and conjunctiva, often related to an allergic or hypersensitivity reaction, which could potentially be triggered by contact lens wear.
Sjögren's Syndrome: An autoimmune disorder that can cause dry eyes among other symptoms. While it's less directly related to contact lens wear, it could be a consideration in chronic cases of dry, irritated eyes.