Why does my singing voice keep changing?

how old are you? if you are younger your voice is probably still developing. you are probably over doing it with your voice. you should consider taking vocal lessons or learning more about strengthening your voice and what techniques and warm ups you can do. Also, sometimes what you eat can change your voice. For example, dairy can change the clarity of your voice because it increases mucus. you may want to gargle salt water to help repair your damaged vocal chords. but definitely consult a professional instructor in some way or another. good luck!