Why is one of my nostrils caving in? (Photos)

What you are noticing has to do with the curvature of the underlying cartilage that supports your ala. It appears from these photos as if your cartilage has a concave curvature. You would need a proper nasal exam to confirm this. This curvature is correctable with surgery. See an expert rhinoplasty surgeon to discuss this more. You are likely a candidate for a limited rhinoplasty to address this. 
All the best, Dr. Brace

Hello and thank you for your question.   This asymmetry is related to your lower lateral cartilage.  Based on your
photograph, you may benefit from a tip refining rhinoplasty. Your surgeon can
accomplish this by trimming, suturing, and reshaping the cartilage in your
lower nose.  Make sure you specifically look at before and after
pictures of real patients who have had this surgery performed by your surgeon
and not just a computer animation system. The most important
aspect is to find a surgeon you are comfortable with. I recommend that you seek
consultation with a qualified board-certified plastic surgeon who can evaluate
you in person.
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

Thank you for your question and photo. I do see what you are referring to. It is certainly a subtle asymmetry, at least based on these photos. Its hard to say if tubes in the nose caused this, probably not. You have to see a surgeon in person to fully assess the nose and discuss together whether any surgery will give you the outcome you are happy with. Best wishes.