Hello and thank you for using JustAnswer Health for your question.
Horizontal lines across the nail bed (left to right) are called Beau's lines. These lines can occur after illness, injury to the nail, and when you are malnourished.
So there are several different situations that might have caused these nail changes in your case.
You specifically report a change in your nutritional status. Particularly if you fasted at any time, this may be the cause of your Beau lines.
Additionally, I noticed when I read your post, that you stoped eating as much meat. That is fine, and is good in those cases where an individual consumed too much fatty meat. But if your protein intake is significanly low, meaning you are not getting high enough protein intake via other sources this may be what is reflected in the nails as horizontal lines.
Other causes include vitamin deficiencies, surgery, and arsenic poisoning. Basically any thing that is stressful to the body disrupting the normal even process of growth.
I reviewed your medications, and Leflunomide has a potential side effect of suppressing bone marrow and the immune system (your doctor typically watches you for this closely by monitoring your laboratory values). If your bone marrow was suppresse, perhaps when you started this medication, a dose was changed, etc., then it might be responsible for the Beau's lines.
As long as your nail growth has returned to normal, this is not something to worry about. One can tell much about the health of an individual by observing the nails beds, so if you see lines forming again be sure and mention it to your doctor, even though your doctor most likely would catch immune or bone suppression first by monitoring your lab work.
If you have questions or need clarification about anything please feel free to respond back to this post so that I may continue to assist you.