What Causes Chest Tightness, Dizziness And Nausea?

I am a 64 year old male who has been treated for high blood pressure for about 30 years. I am taking Labetalol, Ramipril, Hydrochlorothiazide, Amlopidene and Chlopodigrel. For the past year and a half I have been having the following symptoms - chest tightness, dizzyness, and nausea. These symptoms occur when I go from a condition of rest and begin to walk. They last 10-15 seconds and then disappear. Once they end I can walk for several miles seldom with a reoccurrence. I ve noticed them when I am on a treadmill and they usually begin almost immediately. While I am having them my heart rate increases rapidly, usually from low 70 s to around 95-100 in that 10-15 second period. My heart rate then begins to decline and at the same time the symptoms disappear. Usually I can continue a 2 mile walk without a reoccurance. The strange thing is I never have these symptoms when my stomach is empty but almost always will have them after eating. I have had 3 stress tests (all negative) and my doctor has me lined up for a heart catheterization next week. Since they only occur on a full or somewhat full stomach could they be caused by something like an ulcer or other stomach related cause? The feeling is very uncomfortable.