If there are no other symptoms and the rest of the plant is growing fast and healthy, red stems usually aren't a big deal. Often stems turn green on their own. As long as you're continuing to take good care of your plant and responding quickly to any other problems, you should be okay.
If you believe there is a nutrient problem with your cannabis plant, for example if your plant is also growing slowly or the leaves are discolored, then the plant may be suffering from:
Incorrect pH – The pH being too high or too low at the roots is the most common reason to see purple or red stems! (or any deficiency)
Magnesium deficiency can cause purple stems. A good cannabis-friendly nutrient system has a significant amount of magnesium, but if you're using very soft water, or RO water, you may need to add a Cal-Mag to your nutrient regimen.
Temperature – heat or especially cold can trigger red stems in some places
Humidity – very high or low humidity can stress plants (especially young seedlings)
Other types of stress such as light burn, transplant shock, or overwatering can trigger red stems
Sometimes you'll never know why and plants just grow out of it