Putting your plant outside for any length of time will likely burn it if it's only been inside for a month. Your SW bedroom window should be a good place for it.
When it was repotted at the market, did you watch? Did they put the undisturbed root ball into a bigger pot with more soil around it (potted up?) If the soil is all or mostly peat, potting-up like that may cause a lack of oxygen which would impair root function. If what I described is what happened, and the outside of the soil ball was covered with a lot of roots, the plant could be ill from lack of oxygen/rotting roots. Those perimeter roots had adapted to accessing oxygen and are now surrounded by moist soil, which could cause them to rot (and would be diagnosed as overwatering.) Being carried around and driven in a car right after the potting was probably not helpful because it would have caused settling/compacting of the soil.
A pic of the whole plant would be a better indicator of overall health & size, if you are able to add one.
I don't sterilize soil, I just use a more chunky/porous/airy mix that gnats find unsuitable for breeding, bags labeled cactus/palm soil seem to have much better stuff if buying bags. Adding a significant amount of perlite to potting soil can be a workable fix if that's a more appealing option.
Does the new pot have a hole in the bottom so excess water can escape?
