According to the data you provided (nausea, burping, diarrhea, stomach ache), it seems likely that you might be experiencing gastritis. Gastritis is an inflammation of the stomach.


The diarrhea might be present as an accompanying symptom related to irritable bowel syndrome. Gastritis is usually presented with an ache in the upper abdomen that is usually related to meals. The gastric acid that is produced for digesting food is one of the factors that cause the inflammation. Damage to the mucosa is another factor. Presence of certain bacteria called Helicobacter pylori might additionally cause inflammation of the gastric mucosa.


You might want to visit a specialist for the upper gastrointestinal tract for a physical examination, laboratory tests (that might identify Helicobacter pylori), and eventually a gastroscopy which might be the best visualization method that can detect the exact condition of the stomach and the possible cause.


As for the other symptoms like: tender breasts, feeling bloated, and a missing period for two months, they might be related to pregnancy. The hormonal changes that might have been caused by the pill might also cause the previously mentioned symptoms.


However, as these symptoms are considered subjective and cannot be measured, you might want to visit a gynecologist for a physical examination, hormonal laboratory tests, and an ultrasound scan in order to check for pregnancy.