Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the symptoms that you experience.
Various women, when being pregnant, may experience nausea and vomiting, a condition that may appear in every trimester although most commonly in the first trimester. Gastric symptoms may appear without any underlying medical condition due to pressure of the uterus on the intestines, which appears in the third trimester. Heartburn is a symptom that appears in cases of acid reflux, acute or chronic gastritis or ulcers. It also appears in cases of underlying gastric infection caused by Helicobacter pylori.
According to the data that you described, you may or may not be experiencing any medical condition. You may need to have a serological test to exclude possible infection with H.pylori. You may also need to have a gastroscopy (a procedure where a flexible tube is inserted through the esophagus to examine the gastric lining). Consult a gynecologist in case you are prescribed medications for treatment of possible infection with H.pylori.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor for further advice and information about diagnosis and treatment options.