How do you know if they're contractions?

Cara • Expecting our first baby Jan 1st 2016! Married my best friend after 8 years together 10/10/14.
I'm 37 weeks, lost my mucus plug, had bloody show yesterday morning, had diarrhea and upset stomach all night last night. I'm 3cm dilated and his head is very low. My doctor keeps asking if I'm having contractions but I'm not sure. I'm a ftm so everything is new. I have a sore back and butt and my stomach keeps getting rock hard. I don't have much cramping though or major belly pain. I'm confused :-/ they want me to watch for contractions to be my sign to go in.