21 weeks; lower left back pain

Just came on suddenly while sitting. It is sharp and feels like it comes on and off. Not right below my ribs otherwise I’d think it was my kidney, not above my butt, otherwise I’d think maybe sciatica? Is this baby kicking? It’s on and off like it could be but he sits on my right side and it’d be the first time kicking on my left. Is it just organs moving around? Anyone else experience this? Is it something I originally dismissed? Thoughts? Thank you!