I dealt with this the other night during sex and had a few contractions and my belly dropped a bit

It could be from moving during sex. When we do "it" we tend to move our hips, pelvis, and lower back and forth a lot. So that would explain why it hurts. Personally, my lower back hurts after the smallest of activities even bending down too many times. Or when I go work out afterwards it hurts like a bitch