Could have been something on the oatmeal or the chicken.
Did you use brown sugar, or any sugar or sweetener?
On the chicken - any pepper, onion or garlic? Was anything at all added? Frozen chicken pieces usually have at least a bunch of salt water added.
anything else you ate or drank? Bread, salad dressing, onions or garlic in the salad, everything.
Start keeping a notebook - a food log with date and time, what you ate and how much. Include everything! All spices, bread, everything. 1 and 2 hours later - what was the reaction? Even if it was none. If you get headaches you may have to note what happens for 24 hours. It can take up to 2 days for some food triggers to cause a reaction in cases of migraine.
Oatmeal and chicken are unlikely culprits all by themselves.
Have you had this reaction for a while? If it's suddenly come on it's unlikely to be a sensitivity. Same if it doesn't happen all the time, because you probably eat chicken more than once in a couple of months. You could have any number of other conditions - heart, infection ...
If the problem seems more than just annoying you should see a doctor.