Hello, Welcome to Just Answer and thanks for your question. You can take another acid reducer like Zantac along with your Nexium until you see a doctor or go to a free clinic. You should also quit smoking since that always makes GERD, ulcers, etc. worse. They will want to repeat the endoscopy. If you can find it, the liquid antacid Gaviscon contains alginate that helps coat the esophagus and reduce the pain. Here is a list of free clinics. However your best resource may be your telephone directory, since only a small portion of free clinics make the national lists.
It is unlikely to be a tumor, but ling term GERD can cause Barrett's esophagus which can become cancer. Free clinics usually do not have waits at all. You just go and they see you. Try calling another, or go to a walk-in clinic. You may pay $75 out of pocket but it would be worth it. You really can't diagnose anything by what the symptoms sound like. You need a workup.
Anything that painful is serious and needs to be evaluated so you can get some relief.
Tumors may or may not be painful depending on size, type, and where they are.