Usually you start a massage by lying face-down (prone) for approximately 30 minutes, as the massage therapist massages your back, hips, legs and feet. Since your face has been compressed in the face cradle, your sinuses can clog. Some days are worse than others, depending on the weather, your allergies, etc. Once you turn onto your back (supine) the nasal pressure is relieved, causing your nose to get runny. It’s totally normal and healthy.