Sounds ALOT like the terrible twos! Your usually sweet child is acting out more than usual and this is a way of expressing her need for independence along with her frustration at not being in control all the time. Thankfully, this will pass in time. For now, deal with the meltdowns by being patient but firm. Let them think they have some control. My friend who recently survived her son's terrible twos did just that. The little boy would throw a fuss each time he puts on his clothes and before meal times. So she'd let him pick out his own clothes and let him help set the table. Sure, the clothes picked out by a 2 year old will be mismatched and strange and the dinner table will be a mess but she says those are small battles worn and she'll take them for now.
This period, is best to ignore her, it's work like wonder, after 1 round of failure, next time they are more obedient. Make sure you are firm, and fierce, if not they will not scare you and continue throw tantrum. Everytime my girl throw tantrums unnecessary, i will put her behind the door and let her cry till she stop. Make sure u close the lights, so they dun like e feeling, than will know do it again unnecessary.
Wah... Sounds almost like what my girl is doing to us. Haha... We refuse to let her have her way and switch off lights earlier. She can throw her tantrums but we will ignore her. After a while, she knows she can't win.