Many things. Geetha, if you are not dieting and have lost much weight in a short while you should see a physician or clinic. You can take antacids or over the counter acid blockers as a trial, avoid alcohol and very acidic or greasy foods and don't lie flat (use pillows) to keep acid in your stomach. But bottom line is that very fast weight loss can be dangerous and you need to know why this is happening.
Weight loss/pain. Myriad causes, get immediate medical evaluation.
Gatritis. Sounds like gastritis, would stop alcohol and start taking antacids and perhaps over the counter ranitidine twice a day and see how it feels, if no joy in a day or so need to go see the doc. If it kicks up and is very uncomfortable, would go straight to the doc.