I cant get my heart rate up

I work out 3x a week, recently been trying to get back into shape after gaining some weight. I get on the treadmill first, crank up the cardio preset, but I can't get my hr above 124. I'm 25 so my hr should get up to 160ish working out rite? Plz don't say I'm being lazy bc I'm not, physically my legs will cramp from the incline or speed, I'm sweating, but my hr is just not going up.