Could having eaten mouldy bread cause a swollen lymph node in the neck?

Not likely: A swollen lymph gland in the neck would not be related to ingested food, but would be secondarily caused by infection/inflammation somewhere in that region as a rule. If that swollen node doesn't recede after a couple of weeks,if it enlarges or if other new nodes appear, consult your physician for evaluation.

Probably not: if there is a small trace of mold on bread there will be no associated complication from ingesting it. The development of cervical nodes would not be part of the picture. This requires a significant oral Infection Foods that are moldy may have invisible bacteria growing along with the mold. Some ingestion of mold may cause an allergic reaction and respiratory problems.Certain mold can also produce "mycotoxins," or poisonous substances that can make one systemically ill

No: However, everybody has at least one palpable neck node. The ones that are troublesome are obviously abnormal -- rock-hard, or very big, or fixed to something.