Swollen lymph nodes under the armpits can sometimes be a sign of the beginning of an infection. It could be a precursor to a cold, for example. If there are no other symptoms present, it is difficult to determine the exact cause, but it is usually not a cause for concern. You can try reducing inflammation with ibuprofen or warm compresses. If the pain worsens or does not improve after a few days, it would be wise to seek further investigation and consult a doctor.