Am I overweight?

Why am I so heavy? I weight 152 pounds and I'm 5'7''. I'm 30 years old. 10 years ago I used to weigh 20 pounds less. I am not that much bigger now, save for my breasts. I do not exercize (I just take the stairs and walk as often as I can). I am told I look good, not big. My measurements are 40 inches (chest), 29.5 inches (waist), 40 inches (hips), and 22 inches (thighs). My back often hurts and I think it is because I am too heavy, but I struggle to lose weight. I don't even know if my weight is normal, and I'm thinking perhaps I am fat and fail to realize it?