If i see such a patient i will keep the following possibilities:-
1.Since your pain is specifically on the left side of abdomen ,it can be pain due to a renal calculi(stone).For that you might require any ultrasound
2.Second possibility is pain due to heart problem called angina
.Since your age is 56 years, you should get yourself screened .
Meanwhile you can try taking some antispasmodics and see if there is any relief of pain.Avoid taking heavy or spicy meals at night time and take plenty of fluids.
Hope it helps