"I am coming in a while."
Translation:Jag kommer om en stund.
en stund is a noun used about a certain amount of time. (Det tar en stund 'It takes a while') medan is a conjunction meaning basically that two things are happening at the same time: (Jag sjunger medan jag lagar mat 'I sing while I cook')
mellan is a preposition meaning between (Jag sitter mellan två systrar 'I am sitting between two sisters')
om en stund means in a while, but en stund on its own only means a while. However there are some cases where we can use it without a preposition but you might prefer to have one in English. And there's also a bit of confusion about when to use awhile or a while, that may also be a reason for the hint. But I've added a multiple part hint for om en stund nu which I think will make it less confusing.