Hi Kasia, thanks for interesting question.
You are right, normally nouns ending with a have feminine grammatical gender. However, there are exceptions, some of them are masculine. Here are a few examples:
poeta, kolega, sprzedawca, dentysta, kierowca
so sprzedawca is a male shop assistant, the female shop assistant is sprzedawczyni
Thank you for answering!
it reminds me father in Polish, tata is also male grammatical gender! :)