Emotional after dropping niece home, first time Aunty.

My niece is 15 months old and has just had her first overnight visit with me. I am very emotional and have not stopped crying since taking her home last night. I thought it could be that I was very tired, however, today I am still unable to control my tears. I have visited her over 5 times in her life and have never felt this sadness upon leaving. I do feel as though I miss her and I am reminded by all the little things she was doing whilst she was here which continues to make me cry. I do not know if this is normal and I cannot seem to find anything on this, which makes me feel even worse not having any kind of sense where this is coming from. I do not want kids of my own but love that I am an Aunty. What is wrong with me? Why am I so emotional?