How do you know that? When I looked at a couple of sources online, that distinction was not made. Each word has quite a few definitions. For instance, on Wordreference:
se lever: stand up, rise, get to your feet, arise, get up, get out of bed, etc.
se relever: pick yourself up, get back on your feet (again), go up, rise, pick yourself up again, etc.
Larousse, the French definition for se relever, has:
Se remettre debout, sur ses pieds : Aider une personne âgée à se relever.
Sortir de nouveau du lit : Se relever plusieurs fois par nuit.
Sortir d'une situation fâcheuse ou pénible, évoluer vers un état normal, satisfaisant : Pays qui se relève de ses ruines.
Here perhaps the first definition is the closest to what you refer to, as the second is about getting out of bed again and the third is about recovering. But it almost sounds like se relever indicates some sort of real effort, and funny the example also uses the example of helping an old person get back on his feet.
I'm sure the Larousse and Wordreference additional meanings are also true. My answer was based on a) asking my French teacher in my french course (who is also a high school french teacher in Quebec teaching grammar to native french speakers, so I think quite reliable :) ) and b) asking friends who are native french speakers who agreed that those two meanings are the usual meanings in day to day french.
Things are never that simple: http://www.merriam-webster.com/dictionary/alright
This being a "learn French" course (and not a "learn English" one), I'm inclined to think that very common but informal English words should be accepted. It's clear the user was correctly translating the meaning of the French word, there's no need to punish perceived poor-English here.
It seems that there used to be a system where you would have a couple of hearts at the beginning of a lesson (or revision, I don't know since I've been on Duo for shorter time than you have) and you would lose one for each wrong answer. I assume that after you have lost all of them you had to redo that lesson, but that's just a guess.
Ok, that's good. But I was thinking more figuratively, when I say a set back, it could be an illness but it could also be some other misfortune, losing your job for example. In English you can say 'You'll be back on your feet in no time' meaning in general, you'll be feeling better, life will have improved, to return to a state of happy times or prosperity or whatever.....thanks again!!