If you have been given an appointment at the Hospital Eye Service a week after you had your eye screening photos taken, it can mean that the people who examined your photos have seen some changes which need further tests to find out if they need treatment. On the other hand, it could mean that the photos weren't clear enough and you need to be examined using different equipment ( a slit lamp), although it would be unusual for that appointment to be so soon after screening. It should state on your results letter what the reason for your referral is.
If someone is referred to ophthalmology for suspected proliferative diabetic retinopathy, the hospital will try to see them within 2 weeks of the referral from Eye Screening. There's some good information here:-
http://www.diabeticretinopathy.org.uk/proliferative.html (For comparison, if someone was referred for pre-proliferative retinopathy or maculopathy, the hospital will try to see them within 13 weeks of the referral.)
Alternatively, sometimes people are referred to ophthalmology for non-diabetic eye conditions which have been picked up at the Diabetic Eye Screening by chance - some of these conditions will normally need to be examined within 2 weeks of the referral. Again, your results letter should tell you why you have been referred.
In the meanwhile, I would worry only enough to make yourself attend your hospital appointment - you can't assess the situation until you get the full facts. If it does turn out to be diabetic retinopathy which requires treatment, there are plenty of people on this forum who have been through the process and can give you good advice and support.