A 55-year-old female with long-standing rheumatoid arthritis presents with complaints of photophobia, tearing, and constant, severe left eye pain that started last night. She denies any fever, but there is significant conjunctivitis that does not respond to the administration of 2.5% topical phenylephrine. Gentle palpation of the affected eye through the eyelid causes a marked increase in pain. The vision in the left eye is slightly diminished. Her medications include artificial tears, prednisone, atenolol, and methotrexate. What is the most likely diagnosis?