Swollen face and eyes in 2 yr old, ? Allergies

[PJ67]

Hi. My 2 yr old niece has had a swollen puffy face and eyes today. My sister thought she looked tired yesterday but it was only when she got up today that she thought she looked swollen. She took her to the doctor who said possible allergy and to come back if it happened again. She hasn't had anything new to eat or drink in the last few days. It seems to have gone down this evening. Any ideas?
Thanks

My DS had this once, horrendous hives all over his body, we never found out what caused it, he hadn't eaten or drunk anything different and I hadn't changed washing powders, soaps etc. He's never had it again and it was 16 years ago. I gave him piriton and always kept some in just in case. A complete mystery but I remember having something similar when I was 16, never happened before or since.