19 month old suddenly throwing tantrums

Susan • Peyton Louise Jones was born 16th June 2016...Kalen Jones 22nd may 2020

my daughter has suddenly started throwing tantrums. she literally throws herself on the floor kicking and screaming.... I don't know what to do, my mother witnessed it for the first time today and was horrified. she's been doing it the last few weeks and I tell her no, it's naughty. please tell me someone's else's child is doing this and it's an age thing??? she also has been hitting herself 🙈 and she was biting herself but luckily that stopped a few weeks back.