What is going on with mother?

She is 52 and is experiencing a sort of "suffocation" feeling in the center of her chest, I put suffocation in quotations because she doesn't know how to explain it, she just says it's like pressure is being applied to her chest and only breathing heavily can reduce the pressure. She claims it is not painful just uncomfortable. Is this something mild or serious? She thinks it's from stress since she always come home from work complaining about how stressed out she is.