There isn't always an obvious trigger to our anxiety. The anxious response becomes so automatic, our bodies become really sensitised to any physical/environmental changes. Clare Weekes describes this the best for me, she says that over time our nerves get sensitised. I've done CBT and I know they get you to try and identify the negative thoughts that are causing the anxiety and challenge them. I do think this is still useful. But if you can see the obvious reason why you are anxious, maybe there isn't one.
Clare Weeks prescribes acceptance of these feelings, they are a result of being anxious for long periods. They are just the result of an overactive adrenalin system on the body. She says not to add second fear to these symptoms, don't worry about them, just accept them as being harmless and get on with your day to day life. This is quite hard but I really think this is the key to recovery with GAD.