"Estoy como un flan" or "Estoy HECHO un flan" means that you "shake like a leaf or a jelly" because of being very nervous. In any case, and depending on the sentence and the meaning of the conversation, it also might means that you are "very pretty or attractive", saying "flan" as a similarity with "good, nice".

" I'm like a leaf "

In Spain that expression means "I'm pretty nervous"

Esto significa un piropo, lpara decirte que estas "hermosa, atractiva", como un "bombon" like a candy, como una dulzura "sweetness"

Estar muy nervioso/a