The 28-year-old singer's skin has broken out in a rash after she painted herself green to dress up as an alien for the spooky celebration, reported Femalefirst.
"It's Saturday night and I've had a bad reaction to the green paint from Halloween, my face is covered in hives. I'm also sat in a Harvester," she posted on Twitter.
Allen is in the midst of staging a comeback ahead of the release of her new album next year.
"It's not an earnest 'I'm a mum' kind of record. It's empowering. There's some feminist vibes going on. It's the same old me with a bit of swearing going on," she had said.