hi there, yes lymphoma
may present with pain in the abdomen.Do not go to such extreme conclusions just because you have pain. The lump in your armpit however needs evaluation. As you are saying it is soft to hard to touch and painless you will need to get a FNAC done of the same and find out the nature of the lump. Accordingly it can either be excised or other mode of therapy can be given. Do get evaluated for peptic ulcers or gastritis
from your GP and try a course of antacid
as sometimes worrying too much about some thing can lead to stress
ulcers and hence the pain. You need to only concern yourself now with knowing the nature of the swelling in your armpit and if it is something benign then you have to relax and stop worrying till your next appointment with your doctor.Take care.