Why does my left nostri run & feels congested?

Hello. Thank you for writing. 

Generally speaking, your nostrils split their workload. Throughout the day, they each take breaks in a process of alternating congestion and decongestion called the nasal cycle.  If you're breathing through your nose, the lion’s share of the air is going in and out of one nostril, with a much smaller amount passing through the other. Every few hours, your autonomic nervous system, which takes care of your heart rate, digestion and other things you don’t consciously control, switches things up and your other nostril does all the work for a little while. This nasal cycle is going on all the time, but when you’re sick and really congested, the extra mucous often makes the nostril that’s on break feel much more backed up. 

Helena