What Causes Tenderness And Burning Like Pain Under Armpit?

In the last week or so my left armpit has felt tender with almost a burning like pain (though not a severe pain -more like it feels irritated) . I can't feel any lumps, though it is perhaps a bit swollen (though this could be due to all my poking and prodding)....now the right one is starting to feel the same. I am now beginning to stress about lymphoma as I have also been feeling a bit tired and rundown recently (though could be due to looking after three children under the age of five!). I am 38 years old and otherwise fit and healthy, if a bit prone to stress and anxiety.