Da'at Zekenim on Exodus 1:14

וימררו. ובאותו פרק נולדה מרים ועל שם כך נקרא' מרים: וימררו, “they embittered” (the lives of the Hebrews) during that period Miriam was born, her name commemorating that period.

בחמר. בתחלה: בחמר, “with mortar;” at the beginning only this aspect of the labour was conducted with pressure; eventually, everything the Hebrews had to do was done under constant pressure by taskmasters.