Most likely culprit is reflux, especially if its worse after eating and waking you up at night.
As Kathyjelly said, it might be an idea to try a period of eating healthily and logging when you get the symptoms as well as trying to reduce the stress.
You could also try using either Zantac (Ranitidine) or Gaviscon and see if it helps the symptoms.
Fwiw, I have both GORD Gastro Oesophageal Reflux Disorder and Gallstones and I get daily stomach pains, which are worse after eating (particularly unhealthy food) and wake me in the night. Basically, unhealthy food often contains a lot of fat as well as sugar and/or is bulky and difficult to digest (eg bread). This causes the stomach to produce even more acid, which then sloshes around and escapes through the sphincter at the top of the stomach and burns the oesophagus - hence the pain. At night, when you are lying flat, its easier for the acid to travel back up and escape. Lying slightly propped up can help relieve the symptoms.
If it is reflux, your GP can give you PPIs (Proton Pump Inhibitors) which are basically just pills that reduce the amount of acid in your stomach and relieve the symptoms. The most common are Omeprazole and Lansoprazole. Some people take one a day as a maintenance dose, but I prefer to only take them when its particularly bad.
Definitely do the food and symptoms diary though. It saves time if you do get referred to anyone, as its the first thing they'll ask you to do.
Stomach cancer is in fact really rare in the UK, whereas reflux and gallstones affect a large proportion of the population.
Try not to worry.