If you have a feminine noun that starts with a vowel or the letter H, you must use the word “mon” for “my” instead of “ma”. The reason is because it makes it easier to say, and it also sounds nicer to French ears.
mon addition – my bill
mon amie – my friend
mon histoire – my story
If you try saying, “ma amie” and then “mon amie”, you’ll see how the second version just sounds like it flows better.