My catnip plant suddenly developed a purple hue. What causes this? Is it bad for the plant? How can I fix it?

It is a young plant, almost two years old, and has 4 pairs of leaves. The newest leaves have not developed the purple color, but the other leaves and the stem of the plant have a slight purple hue. This happened very suddenly - in one day. I'm concerned about soil conditions, pot size, light, temperature... show more It is a young plant, almost two years old, and has 4 pairs of leaves. The newest leaves have not developed the purple color, but the other leaves and the stem of the plant have a slight purple hue. This happened very suddenly - in one day. I'm concerned about soil conditions, pot size, light, temperature (I'm from Newfoundland and up until now had my room thermostat turned off), my watering practises, or disease. I have not changed the soil/pot for almost the duration of the plant's life span. I water it every couple days (sufficient drainage). It is placed about 2 meters away from a southeast-facing window. Thanks for any insight. :)
Update: Thanks for your answer. :) It hasn't bloomed yet. I water it when the soil is dry, which is actually more like every 3-4 days. It has a very small pot (1.5 inches diameter) and dries out fairly quickly. There are no black spots (thank goodness!).
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