Is my baby allergic to eggs?

I started feeding my 8-month-old daughter boiled egg yolk when she was around 7 months. She loved it and ate them consistently for over 2 weeks with absolutely no problems. She seemed to be digesting them just fine, and she really enjoyed eating them. Well, a couple of weeks ago, she got pretty congested and stopped wanting to eat much of anything. She was coughing, which was making it difficult for her to eat as much. Then, last Thursday, after eating an egg yolk, she threw up everywhere several hours later. And it was violent vomiting that went on for quite a while. So I took her to the pediatrician, who said that her congestion was probably causing her to get sick with solid foods. So for the past week, I've been exclusively breastfeeding. Well, for the past several days, she's been feeling well again, so I tried feeding her avocado yesterday, and that went well. But today, I fed her an egg yolk, and the vomiting happened again! I just don't know what to think because it doesn't make sense that she would have an allergy to eggs that's just now surfacing, when she did fine on them for over two weeks. What's going on?