Stephanie Pratt -- Rushed to Hospital for Serious Spider Bite

Stephanie Pratt had a medical emergency on Thursday -- a spider bite that was so bad, she couldn't feel part of her face ... and not in that fun Weeknd kinda way.

The former 'Hills' star tells us she woke up in London -- where she now lives -- with a nasty bite under her eye, and her face was so swollen it looked like she had a tumor. She immediately went to a hospital to get antibiotics for the swelling, and to drain the puss.

She says doctors determined it was a spider bite, and they believe it could have been a black widow. Stephanie says she was temporarily blinded in her left eye, and lost feeling in her face.