Mommy corner- Allergy woes

In my daughter's 21 months of life, we've never had any serious injuries or sickness, praise the LORD! Last night, as I was giving her a bath, I noticed her lips were swollen. I showed it to my husband and he immediately went to the pharmacy to get some benadryl. Our first thought was allergic reaction to something. He called me from the pharmacy and told me the pharmacist told him we needed to take her to the ER as her throat could also swell up. So here we go, to the ER. As we waited, her left foot and hand also swelled up. We finally saw the doctor and he gave her some steroids and benadryl. We have no idea what caused it. The doctor was saying it was some type of allergic reaction but now we gotta figure out to what... Nothing in her diet has changed... I am very thankful that it was nothing too serious, but the idea of taking your baby to the ER is scary no matter what. Hopefully we can figure out what's causing this soon.