Reaction to cod or dairy?

[HungryDam]

DD has a delayed reaction to dairy (definitely not immediate). She has had cod before and was fine.

Today I gave her Free from Tesco Cod fish fingers which don't have milk listed as an ingredient but have recently taken dairy free off their box. She has had them before and they were fine.

However today, she has come up with an immediate reaction on her face, within 5 minutes. Could it be cod (which she has had before and was fine with) or tiny dairy in the batter possibly (but she's not had an immediate reaction to dairy before either)? Or something else?!

She is 21 months btw.

[HungryDam]

I should add, the only out of ordinary thing she ate about 4 hours before was a strawberry purée