B.D.
I think you're doing a great job with your daughter. :) You handled this so well. Even though there weren't any triggers that you could figure out, toddlers this age, who aren't verbal and can't use any other language to communicate, will often melt down at absolutely nothing, that we can figure out. We have the tools to say, "after my nap, before my diaper change, I'd like to have a nice cup of milk, then a bit of play time, then....." you get the idea. The good news is that she was playing after like nothing had happened, meanwhile, you're traumatized.
Here's my advice, don't take tantrums personally, chalk them up to your child's frustration at not being able to communicate their wishes, feelings, frustrations, etc.
I found it helpful with my son, to help him "name" his feelings and let him know that I was here to help. Here's an example: "I can see that you're (fill in the blank) frustrated, angry, upset right now. Mommy needs to (fill in the blank), change your diaper, put you in your car seat, hold your hand when we cross the street. Don't talk too much at this point, because they can't hear you.
If they're out of control, let the storm pass then about 5-10 minutes later, go to them and sit down, hold them if they'll let you, and speak calmly and in a soothing, matter of fact way. (Don't let them see that you feel helpless or out of control, because that is MORE SCARY for them). You're the experienced, world-wise mommy who can handle anything, even tantrums.
Here's what worked for me. "I can see that you were frustrated,....fill in the blank, angry, out of control a few minutes ago. I can't help you when you....throw yourself on the ground, scream, .....fill in the blank. You have big feelings for such a little person, and I'll help you figure out how to let me know what you need, but it's not ok or safe to....fill in the blank. When I feel frustrated or angry, I make a mad face like this... or I go for a walk or I jump up and down, or.....and then model the behavior. Good Luck.