20 weeks and painful cramps when walking

Okay so I'm 20 weeks and I take an hour long walk each evening. 
This evening though I had a cramp in my uterus that stopped me dead in my tracks. Felt like someone was twisting me from the inside out. It passed after about a minute but then a few min later it happened again. Now I'm sitting down and it has happened twice more.
It's a Saturday evening so I'm unsure if I should call the emergency line or not?