Food sensitivities can develop over time due to a number of factors including imbalanced nutritional intake, genetics, age and illnesses. They usually manifest as digestive disturbances, fatigue, inflammation, pain and dull or irritable states of mind. They differ from full blown food allergies in that testing by your primary care health provider usually does not show allergic responses. Serious or acute symptoms including hives, rashes, diarrhea, vomiting and swelling especially of the mouth and throat should be diagnosed and treated ASAP by your primary care provider or allergist.
The gold standard in food sensitivity testing is a elimination diet, whereby you eat foods that are unlikely problematic (such as salmon, lamb, chicken, sweet potatoes, spinach, and carrots) for a couple weeks to clear out any offending foods. At this point, if food sensitivities are implicated, you should feel much better. Then you systematically reintroduce suspected foods one by one to get a clear sense of how your body responds to them. If you are interested in trying this, please work with a healthcare provider,my self included, or at least read up on it, before you do it.