Hi Guntis,

If you take a look at the following Kwiziq lesson -

https://progress.lawlessfrench.com/revision/grammar/use-mon-not-ma-with-feminine-nouns-starting-with-a-vowel-or-mute-h-possessive-adjectives

You will see that it is because the word ami/e  starts with a vowel and 

ma amie creates a clash of vowels 

Hope this helps!