Interview under caution - PIP

Hi
My husband and I both received a letter on Friday to attend an interview under caution this Wednesday. I have called the DWP today and they've said it's regarding me and my claim for PIP.
I was on DLA from 2013 to June 2016 and was getting higher rate mobility and had a pip assessor visit my home on 6th May. She knew I had a dog because we had a dog crate/toys etc but I said that I don't walk him,we have a dog walker. Anyway I was I was awarded standard mobility and even after MR no change so left it.
I don't know why they want to see me but the only thing I can think of is about the dog as for the last few months I have been walking him myself so Imuch thinking they were suspicious and have been watching me. The only reason I started to walk him myself is because I am getting grief from my hospital consultants to lose weight or I will not be eligible for a kidney transplant (my kidneys only have 10% function). So I've been taking him to a local field where I know other people and their dogs and letting him run around off lead and I either sit on the bench or stand and chat or walk a little very slowly. I also have sjogrens syndrome and Fibromyalgia, problems with my feet and knees and back so I have to take painkillers before the walkand walk for about 15 mins to field in a lot of pain and very slowly and stop for breath etc. I presume that they are not going to be happy with this? It's a vicious circle because if I don't push myself really hard to try and lose 10kg then I won't be put on the transplant list so I'm doing it for my own health.
Anyone got any thoughts? Please be nice.....