cramps - 19 weeks

I'm 19 weeks today. I have had really bad cramps today - so bad I kept having to sit down or lie down. It's not sharp pains, just a persistent ache. It's bilateral, and low. Baby seems fine - his heartbeat is good and he's kicking. But these are the worst cramps I've had in weeks. Anyone else? Is it just stretching? No spotting or back pain.