It is probably a response to the sunlight. Temps, at 40F are also telling the plant to go dormant.
As long as the new growth at the growing tip looks normal, all will be well in the long run. You might consider putting it outside is partial sun during the day and bringing it in at night.
If you had been indoors all winter, and gone out in the sun, naked, without sunscreen, and stayed out all night, naked, at 40F, you would have symptoms, too.