Also, the stems turned purplish. Is that bad? It's sweet mint.

Where is this growing (geographically)? Can you show us a picture?

How on earth do you know the shape of the roots? :) Bothering it too much? Probably.

If it gets sun, it should do fine. Leave it alone for a year and don't water it if it's in the ground unless it wilts.

Is it sweet mint? Or peppermint? Or spearmint?
Peppermint will have pink or purple or red tones in the stems and leaf veins. My experience with trying to grow peppermint in Florida (zone 9, like you) was that it dies in the summer heat.

Spearmint is more heat tollerant.

I've seen different mint varieties and species sold as "sweet mint," and I'm pretty sure there is no specific species or variety with the common name "sweet mint."

How much sun is it getting? Strong, direct sun might affect the taste. I don't have any problems growing peppermint in the summer here, but it can get a bit droopy if left in the sun. It perks up once the shade passes over. Shade during the hottest part of the day is important.

We still don't know where it is growing. If it's Winter or very early Sspring it wouldn't be growing much anyway.