Pus filled lumps underneath the armpit could be a symptom for a number of conditions and it is important to remember the best course of action when concern arises regarding a medical condition is to seek the advice of a medical professional.
Firstly it's worth considering the obvious conclusion that they could simply be spots, or ‘zits’ which appear in parts of the body that have excess amounts of sweat, and given that your armpit is a main contender for the sweat that is produced in your body, getting blocked pores and spots underneath them is not uncommon. A remedy for this would be a cream or ointment and ensuring cleanliness and personal hygiene are at a maximum.
Another possibility to consider is that they could be the result of insect bites. Insects bites can produce some rather nasty lumps. These pus-filled lumps could simply be temporary reactions to a bite that you have received in your sleep
A final possibility is the condition known as 'Hidradenitis Suppurativa' which sounds pretty scary but essentially just comes from the root word 'suppuration' which means the 'formation or discharge of pus'.
Hidradenitis is a stubborn and chronic skin disorder that can affect and cause inflammation of the sweat glands located in both the armpit and groin area. Boil-like lumps, pus-leaking areas and scarring are common attributes which can cause itchiness and discomfort for the sufferer. The condition can often be difficult to get rid of, and unsightly and embarrassing for the sufferer but luckily can be treated with either medication or surgery, so if you have concerns regarding your condition, the best course of action is always to consult with a doctor or medical professional.