It sounds like the soil previously used had problems, LasVegan. A plant can't make the soil turn bad. Or the stuff it was in before was not good, (or turned bad due to years of being in the same pot) and that's why it had such a small root system.
Not all stuff bagged as 'potting soil' is of good quality. It should feel light and fluffy (although some decent soil is sold moistened and the bag can feel heavy).You can probably buy Miracle Gro potting max anywhere and that should be good enough.
I still vote to repot. Loosen the roots and make sure the new soil gets poked in to fill the spaces.
