Last week I found a small tender lump just below my right elbow. Today I found a small tender lump in my left armpit. Any ideas what could cause this?

Need to examine. It is not feasible to provide a meaningful opinion without examining you and your symptoms warrant evaluation by a physician. The lumps may be enlarged lymph glands. You may watch and wait. , if any of the lymph gland doubles in size, becomes painful, ulcerates, or other nodes become enlarged, or you develop fever, or lose weight, you should see a doctor.