Lower back pain

[msincredible]

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

The pictured position will stretch your abs and crunch your lower back, it's not surprising it hurts.

Some of the stretches shown here are good for lower back:
http://websterbicycle.com/page.cfm?pageID=80

Did something change about your bike fit? Maybe your handlebars need to be adjusted slightly.

Strengthening your core muscles will also help your lower back.