Hi I noticed a lump (smaller than a pea) in my armpit 6 days ago, it got very smaller but I can still feel it sometime. Is it bad?

Lymph node ? Possibly a lymph node. Have a breast exam and check by your doctor. There are lymph nodes in the axilla(arm pit region). Getting smaller, probably a reactive node. They become enlarged as a response to nearby inflammation or infection. If you shave in this region, a small nick in the area can cause a response. Breasts are nearby, sometimes a sign of problem especially if it does not resolve.