Call from DWP ref PIP

Hi, I have just had my review , I was granted a home assessment As i have just come out of hospital after heart surgery. I was called last week by the dwp (They said that there was nothing to worry about) and asked when I was in hospital in 2016/2017 and how long I was in. I was only in for a few days at a time. What. I am confused about is why are they going back looking at the amount of time I spent in hospital from a few years back when they have reviewed this last time. They DWP told me I must inform them every time I go in even if it’s just over night. Has anyone heard of the DWP going back over old claims like this.