If something comes on suddenly, I'll usually suspect dietary first.
Most stuff in your life is constant but the one thing that changes on a daily basis is what you put into your body.

You're definitely right about eliminating everything from your diet and then slowly reintroducing stuff. It took me years to finally figure out that my constant stomache pain was caused by peanut butter, eggs or fried onions.
Even longer to figure out that freshly ground pepper gives me symptoms like asthma or dill pickles giving me major nightmares. (seriously man... a coupla pickles before bed and I have some really intense, disturbing dreams.)

Sometimes it's fun to experiment. Stick to nothing but cheese sandwiches for a week and then eat a big salad on the weekend.
Holy Crap!! Literally.

Medication can also have a really delayed effect. If you've been prescribed something for a long time and suddenly go off it, it can take a full week before you feel the symptoms.