Hello, With your history of the recent sinus infection and nasal discharge, I suspect this to be related to the infection. Possibilities include strep, thrush, or virus. Your saliva or nasal discharge may carry the infection to the corners of your mouth. If you were breathing through your mouth due to sinus congestion, saliva or drool especially at night can cause the irritation. Did you take any medication for your infection such as an antibiotic? Some antibiotics such as amoxicillin can throw off the natural balance of the oral bacteria in your mouth and cause thrush. If strep is untreated, you may need to address this to help the corners of your lips clear up. You can try various over-the-counter treatments and gels to ease your symptoms. A light coating of Vaseline may offer a layer of protection to allow healing. Applying vitamin E can assist healing. There are anti-viral gels that can accelerate healing. You can consider a visit to your physician or dentist to help with a diagnosis if you did not see a doctor for your sinus infection. Depending on your diagnosis, a prescription medication or gel miay be available. Other causes include a collapsed bite and vitamin deficiency. Your dentist can examine your bite. Your doctor can order blood tests to check for a vitamin deficiency. Make sure you stay hydrated, have a nutritious diet with balanced vitamins, and that your dental health is maintained. Take a decongestant if necessary to stop breathing by mouth. Avoid stretching your mouth open wide and do not wet the corners. I hope my suggestions offer you some relief. Thank you for your inquiry.