My wife was getting a shower ready in a guest house for my daughter and noticed a small, flat, thin , black bug with twi antenna and two tails. She couldn't see any legs but it was fast and seems to wriggle rather than crawl. When she picked it up with a tissues to put it outside it bit her on the thumb, Within seconds the thumb swelled up. After 24 hours most of the swelling has gone but her thumb is black and blue
Any clues
Location: North Wales, UK