15 month old having meltdowns constantly!

I will preface this with saying that I have twins, they throw tantrums and I know the difference between their normal "I want what I can't have" tantrums and my daughters new "I'm just angry and screaming" meltdowns.

I do not know why but this started just recently and I don't know how to handle it. Until about two weeks ago she was the happiest baby I'd ever met, dancing and running all the time and loved to learn fun new things (she's a pro at the itsy bitsy spider song) but now it's honestly as if she's possessed. She has been absolutely losing it every single time she doesn't get what she wants right away. The tantrums aren't even something you can solve by giving her what she wants, it will continue until she has been effectively distracted, usually by going outside or just in another room. She throws herself back no matter where she's at, screams absolute bloody murder but has almost no tears at all, kicks at things (or even people sometimes), smacks herself in the head, bangs her head on the floor, and won't let anyone hold her because she tries throwing herself back even more. I have never been worried about her behavior before, she makes eye contact and learns quickly, loves to play and read with us, she can even almost dress herself on her own. I just need some advice or to hear that this is normal maybe. Even my mom says she's never seen a 15 month old throw tantrums this bad. They just had their 15 month checkup too and the pediatrician said she was so pleased with how my daughter was doing, everything was perfect and she had no concerns at all. All I've been able to do during her meltdowns has been to let her have it out in a safe place where she can't hurt herself, or carry a thrashing child outside so she forgets why she started the tantrum in the first place.