Dull burning ache in the central/ left center part of abdomen at the bottom/below ribs? Off and on for a month. Stomach acid?

Pain. could be. Try OTC Prilosec, if better it will serve both diagnostic and therapeutic treatment. If not, could be costochondritis too.
GERD. The location of your pain is consistent with "stomach acid" known as gastroesophageal reflux disease (GERD). When acid refluxes up into the esophagus, it burns the tender lining of the esophagus causing pain. This can result in serious problems over time, cause as narrowing of the esophagus, ulcer, bleeding, and even cancer. Medications that lower the acid content in the stomach can be helpful.