[Sparkly*Serenity]

Piriformis m. stretches perhaps? The sciatic nerve *easily* gets peeved off in the area that you describe. On back, cross bent right leg over straightened left leg leg and gently use left hand to bring it over. Can also cross right leg over bent left leg and then let both knees stretch over to the right in a twist ("crocodile stretch"). You can also do a search for these b/c they're pretty common.
I would also check on your level of hamstring tightness and crank up the strengthening efforts on ab and glute-predominant ex's. Squats are awesome so I'm glad you just went down a notch vs. quitting untl you felt better. And sitting too long in one position seems to fire up low back issues in folks that have previously had...issues in this area.