There are two forms of diabetes. Type One usually comes on during childhood, occasionally later, and is an auto-immune disease resulting in the body attacking and disabling the pancreas. The outcome is an inability to produce insulin and shuttle digested sugars into the muscles that need them for energy. No insulin eventually results in death, and in Type One, this can happen in a fairly short period of time. A common side effect is production of huge amounts of urine as the kidneys attempt to dump some of the sugar from the blood. Thirst, hunger, weakness and weight loss are common, too. Insulin must be injected daily to maintain life.
Type Two is usually later onset, though many obese younger kids with poor diets are becoming victims. For this one, you need a diabetic gene that disrupts your body's ability to regulate how much insulin is released in response to eating carbohydrates, resulting in too much insulin being released too often, in essence "wearing out" the cells that produce that hormone.
Very early (pre-diabetic) Type Two symptoms include blood-sugar crashes (hypoglycemia), with weakness, headaches, and light-headedness. Later on, the body becomes insulin resistant and it takes more and more insulin just to ferry the sugar from the blood into the cells, further taxing the pancreas. Eventually, it stops making insulin from overwork, and insulin is needed as injections.
Since you are young and fit, Type Two is unlikely, but not impossible. If you are genetically predisposed, you can still get that. But either form of diabetes is dangerous, and you really should see your doctor right away if you have any reason to suspect this is your problem. Wishing you luck.