Why has a lump formed in my nostril under skin graft on my nose?

I suggest that you leave the area be for now and wait to see your surgeon at your follow up and discuss it then if it hasn't gone down. Thank you for your question and happy healing!

Most likely it's just from the trauma of the surgery and is either swelling or a tiny pocket of blood. Leave the area alone. I'd assume you're going to see your surgeon in the next few days. Have him/her look at it to see what it is and determine next steps. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."