I was wondering if I should see my doctor? I have discomfort under my left rib, little red pin sized dots, some in mouth, very tired. Cancer?

See a DOCTOR. Not necessarily cancer, but could be any number of things. Without having any other information (and this is very little) my initial thought is to be concerned about your spleen (underneath your left rib cage) and your platelets (could cause little red dots called "petechiae"). An enlarged spleen and low platelets could be a sign of lymphoma, autoimmune diseases, itp, infection or others.
Please do ! It may be simple rib or muscle problem. We do not know your age, gender or other history. What are these dots; infection? Viral, bacterial? Just one visit, with good history, blood count, glucose, liver and kidney function might relieve your anxiety and ours too ! get on the phone and get the appointment today ! send us a follow-up so we can be of further help.
You should. Dont waste any more time. You should see your md and have a proper evaluation. It does not have to be cancer, it could be caused by some other condition. You will not know unless you see you md and get yourself checked.
See your doc. These can be symptoms of blood cell disease and cancers. Go see a doctor!