Okay so I'm probably gonna ask a stupid question but how do u know when ur having contractions? And…

Okay so I'm probably gonna ask a stupid question but how do u know when ur having contractions? And how long do I need to wait before going to hospital? I'm 38 weeks pregnant and having bad cramping in my whole stomach and my stomach gets hard and then soft. I can't lay down for long without feeling sick, and my back hurts alot