Stopped saying words?

Had anyone’s LO stopped saying words that they used to say? My daughter has said dada forever, and started saying mama around Mother’s Day. She mostly said it when she was tired. For the past couple weeks, she has completely stopped saying mama. She will say dada all day, but when I ask her to say mama, she shakes her head no. She’s recently learned to shake her head no and started saying uh-oh. Is it normal for them to drop known words when they learn new ones? I’m not super worried, but I assumed she would just add new words instead of replacing them.