Your armpits can feel like this if you are dealing with an infection - similar to the glands in your neck becoming enlarged with a throat infection. It is unusual - although not impossible - for breast cancer to cause any 'symptoms' like pain, achiness, etc. until very advanced. So you are probably worrying needlessly. However, it will do no harm to have an examination by a doctor, just to put your mind at rest. Mention your fears about your genetic/hereditary risk. At 18, again although not impossible, breast cancer is very unlikely. There is nothing silly at all about being concerned - see the doctor, and then relax.