It's entirely possible that this is perfectly normal. Exercise, in particular cardio, stimulates certain hormones and suppresses others:
A one-hour aerobic exercise session helps reduce ghrelin levels and increase Peptide YY levels, decreasing your appetite when you are finished, according to a 2008 study published in the "American Journal of Physiology."
Generally, this loss of appetite recedes within an hour or so of your workout. In fact, a common problem among people working out is that, when the hormone levels reassert themselves, they can do so with a vengeance, resulting in eating more than you had intended as your body tries to refuel itself. If it's a more longterm problem, particularly if you're finding yourself getting woozy from low blood sugar and if you're doing an excessive amount of exercise, then you need to take it more seriously, finding some easy-to-eat foods that you can eat an hour or two after exercise to ensure your caloric intake.
As with many things diet-related, one of your best first steps is to start writing down what you eat. That will give you an idea as to whether you're still getting in the 1200 calories (1000 for women) that the FDA recommends as a minimum.
Under the heading of anecdote, I've personally found that I seem to eat less overall since I've started running three times a week. I just don't feel like I need as much food. However, that's more from the perspective of snacking, and smaller portions, than not feeling like eating all day.