I woke up with pain in my upper stomach right in the middle under rib cage.  I only hurts when I walk.  I hold my upper stomach to help it feel better.  The pain goes around my sides.  I have had this before when I had mono.  I have experienced this pain several other times but it goes away after a couple hours.  I have had the pain for 12 hours now.  I feels a bit better now but still there.  Should I see my doctor?
Reading the details mentioned by you, I can think of two possible causes. One is constipation which could lead to such a pain. The other and most likely cause is gastritis or gastroesophageal reflux. It could be that you are having hyperacidity and the walking makes the acid reflux and hence the pain. Eating small but frequent meals should help in getting over it. In any case, be sure to consult a doctor and get it examined and treated.