how to say "on" in this case?

how is "on" said in this sentence:

"she is on fire"

i know it must be a stupid question, but my mind is stumped on it.
the girl is NOT "on" the fire, but, instead, "in" the fire, so how would you say this sentence?

She's on the fire just after you put her there, like you would a witch or, perhaps, a Catholic.
A little later she'll be in the fire, where, some would say, she belongs.
She's on fire means that a) her flesh has combusted, or
b) She's got the hots for you whitopian1

"Cała płonie" would be fine for both instances.