Angel to Monster

My 3yr old daughter has recently started to turn from an angel to a monster in a matter of seonds without any warnings. Every child has tantrums but I'm starting to become concerned. Everything will go from nice to disastrous and it usually ends up with us having to restrain her. Today, I had to go into work unexpectedly for 4 hours and had to take my daughter because my husband was already working. She was awesome. I took plenty of movies and toys to keep her occupied. She ended up playing with a coworker’s daughter. Both behaved well with little reminding. They were right there beside us and we interacted with them while we worked. On the way home I praised her for her good behavior and made a quick stop at the grocery store. As I was bringing the last bag of groceries into the house she came up behind me and bit me hard on my back. I spanked her and told her to wait in the living room while I put groceries away. In the middle of putting them away she threw the stool she uses to wash her hands at me. She just started throwing anything she could get her hands on, laughing the entire time. When I thought I had the situation under control I went back to the groceries. She came up behind me and hit me with her stool as hard as she could. I had to fight to get her under control and sent her to her room. These bouts of violence are very disturbing to me. Is this a sign of something more serious?