Post-operative swelling can cause congestion, making it difficult to breathe after surgery. This is something that resolves over time. Post-op swelling can also give the appearance of asymmetry and irregularities. 18 days is still soon after surgery, and you will need some time to heal. I would recommend following up with your surgeon regarding any post-operative concerns, as he/she knows the details of your procedure and will be able to advise you. I hope this helps, and I wish you the best of luck.
After a septorhinoplasty, it is not uncommon to have swelling that limits some airflow and makes you feel obstructed. This should get better with time.
Resolution of internal swelling after septorhinoplasty will depend largely on what was done during rhinoplasty surgery both externally and internally - concomitant septal or turbinate or sinus surgery. Internal swelling and congestion typically improves a few weeks after surgery but can take up to 3 or 4 months to improve significantly. During this recovery period, saline nasal spray, or steroid nasal spray may be necessary to obtain temporary relief from symptoms until full recovery has occurred. You should discuss if this is required with your surgeon during your follow-up visits.