My cat is mad

Molly cat has gone mad. Seriously. I don't know what the problem is, but she clearly has intermittent insanity. Let me explain:

This morning, at 0500, I was awoken by a very cross cat. I refused to take any notice. She was incandescent with rage, pupils enormous, biting at me, scratching any bit of me she could reach. I wrapped myself in the duvet and refused to come out. Within minutes, I could hear her racing around the house, bumping into furniture, chasing any shadow that could be seen. She was in and out of the window to the terrace and, when I finally got up, she had clearly been bringing in leaves - the bedroom floor was covered with them.

Two hours later, when I actually woke up, she was fast asleep against my legs, looking as if butter wouldn't melt. She leaned against me when I sat on the edge of the bed, purred and demanded attention. We had a lovely ten minutes before I went through to the kitchen to feed her.

I decided it had been an aberration and forgot about it, but this evening she was again furious, biting at me when I tried to pet her, refusing to come to me. More worrying, she looked at me as if I regularly beat her up (which, I assure you, I don't) and ran away every time I made overtures.

She has also taken to ripping up envelopes. So far, it has been mainly open ones, but today I found her chewing on an unopened envelope!

PLEASE experienced cat people: what is going on and what should I do about it?

PS She is now sitting on the keyboard intermittently, so any typos are hers!