Guys, we are learining Polish here, not English. The English phrase "she is under the shower" translates the literal meaning of how Polish people say it, it makes you REMEMBER the Polish phrase. I bet you'd have harder time memorising the phrase if not for this seemingly erroneous translation.
Just try to split the notion of "shower" in your head. It's a shower-place with a shower-head giving out shower-water. For English speakers the default meaning is the place and the water (IN which you are), for Polish it's the shower head with the water (UNDER which you are).
I don't want to downvote you, but I am willing to bet real money that no native English speaker over the age of three has EVER said they're "under the shower". If we're talking about the sensation of a shower outside of your bathroom, like a rain shower or being showered by droplets from a car passing over a puddle, it's "in a shower" or even more likely "caught in a shower". Never have I ever heard "under the shower", but "under the showerhead" would be perfectly valid.
'Under the shower' is most definitely possible in English, and was my first answer, because it seemed more literal and also is used in English.
Imagine the doctor has prescribed some kind of special shampoo for you. The doctor could - would probably - say, 'When you are under the shower, get your hair completely wet and then apply the shampoo,' for example.
More than 25 years as an international teacher of English and with two degrees in literature - I've definitely come across this in the US and in books over the years, though I think it's not very current now. It may be more current in places where freestanding shower boxes are not common.
It would be the obvious thing to say when showers first became common for washing. One is 'in the bath' because one gets 'in' the tub, 'in' the water. But originally, one would stand 'under' the shower of water - the natural way to think of this novel way of 'bathing.' In time, with enclosed shower cabinets (rather than shower heads in bathtubs). one would have more of a sense of being enclosed in the 'shower box' so to speak. Hence the proliferation of 'in the shower' rather than 'under' it.
A clue to these things is that there is a logic in other languages saying - and staying with - 'under' the shower. Especially when you consider that in Poland, most 'showers' are still just a shower head on a long hose mounted to the taps in the bathtub. Freestanding shower units are only becoming more common in the past 10 years or so in Poland. So they retain the early sense of standing 'under' a shower of water, a sense that has faded - but still exists - in English.
I wasn't aware that this phrase isn't considered proper English, or that it's so rarely used in the wider English-speaking world, so I'll accept that it may not be an appropriate translation to mark correct here. However, it is a phrase that is said by native English speakers to mean the same thing.
Most often I've heard it said to describe the act of getting under the actual spray of water, as in "She's getting under the shower now", "I was just getting under the shower", "Hurry up and get under the shower", and so on. It's closer in meaning to "taking a shower" than it is to "in the shower", because you can be in the shower without taking a shower.
I put this question to the English stack exchange. http://english.stackexchange.com/questions/304448/is-she-is-under-the-shower-a-proper-english-sentence
Because this will not ever be said. You can be "w kabinie prysznicowej" (in the shower cabin), but absolutely not "w prysznicu". That's physically impossible unless you are Ant-Man and can really, literally fit inside the device.
As you've seen, I inclined towards leaving "under the shower" as a literal translation, but the arguments against it were serious enough. So we shall treat it as one of those common situations when the languages just use different prepositions.