CB, you need to CALM DOWN! Pretty much every symptom you have reported, aside from the cough, from gaining weight to losing weight to digestive problems, stomach acid and pain, can all be caused by STRESS! Your constipation could well have caused you to gain weight. Now that things are moving again, you should naturally drop weight. The jeans could be loser just because you are no longer bloated. For that matter, you upped your water intake because of the constipation and the more you drink the less likely you are to retain water - so you could be losing water weight. You've complained of chest pain in the past. I actually ended up in the hospital and had a cardiac cath once after complaining of chest pain...which turned out to be acid reflux! Your stomach and chest pain and your vomiting acid could well be acid reflux. After all, you lay down at night and it creeps up into your esophagus.
As for the cough...it's the end of winter. Stuff's going around. It happens.
Constipation is highly unlikely to be colon cancer. Stomach pain is highly unlikely to be stomach cancer. Coughing is highly unlikely to be lung cancer. Cancer is NOT the first thing you should be thinking of!
Stress can make you sick. Stressing out over every symptom just makes you sicker.
You are going to be okay!!!