Lower back pain

[Ana]

I have recently developed lower back pain when I'm cycling on my road bike after 5 miles or so and when I stand for long periods of time. I spent a large amount of time in the drops because it relieved the back pain...

I also tried a stretch (pictured) which I had previously been able to do comfortably but now it's incredibly difficult and painful.

I have stopped weight training but I've never experienced something like this.

Any thoughts?