How do i know if im having a contraction.?

last time i called the dr they said go to the hospital cuz it was a contraction but when i got there the monitor said it wasn't and they sent me home felling like and idiot. now it just feels like a very uncomfortable pressure in my whole stomach but not painful what do you think is and if its not then what should it feel like? (im 36w5d and messering at 40w)