Firstly, if you do have diabetes it is not the end of the world, it is actually a potential new beginning. A 6.8 is relatively low, at one point I was over 16. That being said a potential diabetes diagnosis is a shock. You are the same age as me and despite your physical activity it is easy to develop Type 2 (I am assuming this at this point, but you should get the second test). There are many different body types that get diabetes, you might be a TOFI (thin on the outside fat on the inside). There is a concept called the PFT, personal fat threshold, which is individual to all. Some pack visceral fat around the organs which can cause diabetes, drugs such as some steroids or statins can also inflict this disease. Over time the insulin that your pancreas creates to drive blood glucose in muscles and cells, no longer can perform this function (like a dam breaking), your body produces more insulin, but "insulin resistance" has built up, like the story of the boy crying wolf, your body doesn't / can't listen to insulin signal, so glucose remains in the blood stream, which equals diabetes.
Unless you were actively looking to avoid diabetes, you may have eaten past your tolerance for carbs over your life so far, it is very easy, even with so called healthy foods (breads, rice, pasta, potatoes and derivatives), I had very little junk food, but ate the things in brackets and lots of fruit and fruit juice, along with oats. Have a read of the below:
http://www.diabetes.co.uk/forum/threads/basic-information-for-newly-diagnosed-diabetics.26870/
Once you have your second test, you will know where you stand. If you do have diabetes, changing your eating pattern is the easiest way to either reverse or put diabetes into remission. If you are an omnivore, then meat, fish, shell fish is zero carb, so can form the basis of your potential diet, low carb vegetables work well (cauliflower, broccoli, greens, asparagus, green beans, courgettes, tomatoes are good choices). Salads without the dressings are worth a look. Any fruit ending in berry works better than tropical versions. The workouts you do with food choices, will help with insulin sensitivity. Lots to take in but carbohydrate intolerance is almost par the course now, as the standard guidelines promote circa 60% in the diet, many of us are around 10% after the fact.