"I changed the bedsheet."
They are gender variants. Zmieniłam is female speaker. (an usually -łam ending) Zmieniłem is male speaker. (and usually -łem ending)
we have this distinction also with you singular, we ,you plural. on/ona/ono have masculine/feminine/neuter variants oni/one have their own respective variants
take a look on the chart to see what I mean https://en.wiktionary.org/wiki/zmienić