This is terrible ,the same thing happened to me some years ago,I was called for interview ,be prepared it's quite intimidating ,it feels like being interviewed by the police,with the interview being recorded and verbal warnings about evidence being used in court.
I just told them the truth,they certainly try to catch you out ,they had been following me for quite some time,and had clearly sat watching me outside my house.
The next problem I had was getting to know the outcome of their investigation ,I heard nothing for months ,when I rang them all I was told was it's still under investigation.
Finally I got my MP involved and he got a result in less than an hour,they had decided there was no case to answer.
I certainly had the impression that the people who interviewed me needed a great deal more training in disability awareness,I thought they were of the impression that if you were not a wheelchair user,you were not entitled to DLA.
I would advise that you take medical evidence with you,consultants letters etc,and even consider starting a diary now about how your illness impacts on your daily living.
You are allowed or at least I was to take some-one with you ,but they are not allowed to speak on your behalf,unless you are wealthy then you can take a solicitor ,which I think is extremely unfair.
Try not to worry about it,I tried to look at it this way ,I am receiving a lot of money from the government because of my disability its only right that they ensure it,s going to those people who deserve it.
Best of luck with it.