My cat is suddenly afraid?

Hello,

My 3 year old cat has been hiding in closets and under beds since yesterday.

She's normally outgoing and social. She doesnt have any medical issues except for being overweight, which we are addressing. I adopted her from the shelter 2 months ago and she settled in eagerly, with no hiding. She greets every new person, and my roommate and me when we get home.

Yesterday she stayed in my closet almost all the day and ate her food, whereas today she hid for most of the day, and ate some breakfast when I put her in front of it. When I made a loud noise, she got up and walked back under a bed, which her back and tail low to the ground. She didnt eat any dinner or drink water when I put her in front of them. Normally she is a hungry girl who demands food. She also hasn't used the litter box since yesterday.

She'll let me pet her under the bed and purs. She even goes as far as to put her paw over my hand when I withdraw it, which is her way of asking for more petting. She meowed once in protest when I picked her up, but she meows most times I have picked her up.

I can't tell if she's in pain or just scared. Nothing in our house has changed. I'm going to call the vet if this doesnt change tomorrow.

Any advice?