It could have just been a different part of the chicken meat like a tendon or something.

My Mum once ate a mouthful of chicken in a restaurant and it turned out to be red and undercooked in the middle. She complained and was offered a refund but said she was very worried about food poisoning- but she was fine.

Basically I don't think your food was undercooked but even if it was then it doesn't necessarily mean fp!