Downward Dog Pain

That sounds like a weakness in your back or abs. No worries, most women have said weaknesses. But, you should stop doing those moves that hurt. That transition is very hard on the lower back. Perhaps you can try it with your knees on the floor, to relieve the pressure on your lower back. Otherwise, go seek out a certified yoga instructor to tell you what you're doing wrong!

Well, maybe I phrased it wrong. The pain only happens when I transition into the downward dog. As soon as I move back into the plank or upward dog, I'm fine. It's just that split second in a transition.

And my workout was a light kickboxing cardio workout. I've been doing it for months so I didn't put any stress on my body when I was doing it.

No, you phrased it perfectly! You did a workout, then you did Yoga.

The movements involved in most kicks activate the hamstrings. Depending on the flexibility of the hamstrings and your form during kicking (and form can go south when kicking in a cardio-type workout) you put stress on your lower back. They are all part of the same chain. A lack of core activation during kicks can also overly stress the lower back. Normally, you wouldn't have noticed it, done a light warm down and got on with your day.....

However, adding the Yoga, probably pushed the lighter repetitive strain caused by the kickboxing into something a bit deeper. When you transition to downward dog, you are activating the same parts of the posterior chain as when you kick, but in a slower, deeper more isometric manner. A lightly damaged muscle can be further damaged, or torn, under this deeper isometric stress. If you are not used to Yoga, I would do it on it's own for now. It has an image for being airy-fairy and an easy workout, but in fact is a difficult and hard to master discipline.....