Sore back

Hi

I did front split stretch 4 times(last was yesterday) and i never had sore back from that, but this morning my middle back was sore.

So my question is what cause that?

The one that jumps to mind is having APT in the wall single leg stretches.

Or if it's more like a DOMS pain you could have a lot of weight on your hands in the single leg pike.

Not sure what else it could be. What else did you do that day?

That day I only did some warmup(mostly legs) and front split series, but I was tired after 45 min. which is also 1st time. Last 3 times i only had sore calf and quads and I was able to complete full 45 min. without any problem.