A few weeks ago I planted Spinach in a couple of locations. One is in a 24 inch planter. All of it is doing fine, but the container crop seems to be flowering. There are no more than three sets of leaves and now they are getting a tight bunch of, something. It's a tight short vertical stem growing in the center of the plant. It is a stem with tons of very small, very tight circular things. Could the plant be flowering? On the one plant where I removed the stem there are now two stems growing. Interesting, but I am going to want to eat this stuff.

You don't mention where you're located, but since it seems to be hot everywhere this is probably your problem. Spinach does NOT like hot weather and will bolt (go to seed) even when it's small.

I have had very young plants go to seed before getting big enough to pick.

And some varieties are more prone to that than others.

Could it be that you planted Strawberry spinach? They set fruit up the stem and the plants don't get very big...at least they don't here.