Painless contractions

Have any of you had contractions and not known? Everyone keeps saying oh you'll know when you have one. But the doctor asked if I've had any yet today at my appointment (I'm 37 weeks 3 days) and I said I've just had some light period cramps but idk if that's a contraction. Well I was hooked up to a moniter for our babies heart beat and the tech said you your getting a contraction now and I had no clue lol I'm worried I won't know when I'm in labor