The Retinal Eye screening that we diabetic's regularly undertake is the first port of call in examining eyes on a mass basis, and they may well have picked up on some minor thing that they would want to have investigated further by a more senior eye specialist.
Now that doesn't necessary mean that there's a serious problem with your sight ,only that they want their initial suspicion of whatever it might be confirmed by a more senior eye consultant, so don't get yourself too worked up over this, and simply look upon this as being a good thing that whatever they suspect has been found early on and not left to worsen over the years to come.
It might even be that they have detected the very early signs of say a 'Cataract' developing, which again is nothing to unduly worry over. I myself have had both Cataracts replaces together with a number of various eye operations over many years and am currently under sentence of needing a major operation to both my eyes sometime in the future which only has a 1 in 5 chance of success and I'm not unduly worried.
In the majority of cases patients are told that there is absolutely nothing to worry about and they will simply keep (pardon the pun) an eye on the situation, so finger's crossed that this will be the case.
Best of luck at the eye clinic hospital appointment. Please let us know how you get on..