Mildly bitter is normal for plants stressed by temperature and moisture fluctuations. Growing in the Southern portion of Florida during winter is a prime example of this. It rains then months go by. It frosts then we reach an 80ยบ high the next day.
However, if you come across an EXTREMELY bitter cucurbit (luffa, zucchini, cucumber, squash, melon, pumpkin...) spit it out and don't eat it. Doing this might just save yourself a day or two in the bathroom.
http://cuke.hort.ncsu.edu/cucurbit/cuke/cukehndbk/cukebitterness.html
