^Agreed, your betta needs a heater. At cooler temperatures she will have more trouble digesting food which in turn can lead to constipation and chronic bloat. Try to pick up a heater asap.
Otherwise, Hikari isn't the best food. It is high in wheats and low in meat so it is known to cause constipation, impaction and bloat. If you can, pick up a new food like New Life Spectrum or Omega One Betta Flakes.
For now, treat conservatively with Epsom Salt at 1tsp/gal to see if there is any improvement. If there isn't don't worry too much. It sounds like it might be a birth defect.