Thanks for posting on HCM
From your description you are presenting a developing abscess
under your armpit which is usually due to bacteria infection like staph and you will require a course of antibiotics such as amoxicillin+ clavulinic acid. The pimple you are describing might be a folliculitis
. Antibiotics prescribed might be helpful too.
You can get your gynobs to examine you first for the abscess and the pimple before you take any medications especially as you are pregnant.
Hope this helps