Why is there a red patch under my dog's eye? Skin or eye infection?

Thank you for using Petco Pet Education Center, formerly Petcoach! I'm sorry that Bella isn't feeling well - she's such a cutie! It looks like she has some hair loss and redness on the lower eyelid. This can be due to an infection of the skin, or because she is scratching/pawing at her eye causing hair to fall out. She should definitely be examined by a veterinarian as soon as possible, because eye conditions can quickly turn into emergencies! The itching in the area can be caused by dry eye, which can cause an ulcer in the cornea, conjunctivitis, or eye pain for an number of conditions. To keep Bella comfortable until she is seen by a vet, you can get her an E-collar (or "cone") to prevent her from scratching/pawing/rubbing the area and causing further trauma. I hope Bella gets better soon!