I have swollen painful/itchy lumps under my armpits, mainly left hand side, following 3 weeks of long and exhausting travelling. is it just due to fatigue?

Hydroadenitis: You should make sure that you do not have a condition known as hydroadenitis which can be provoked by using deoderants that contain aluminum. If you switch to a non aluminum formula and the problem persists you should check with your primary care provider.

Not due to fatigue: Your axillary symptom is not due to fatigue. Were you wearing a backpack whose straps might have irritated the area? Were you sweating much so that you might have developed a yeast infection? It's possible that your axillary symptom is completely unrelated to recent travel. You'll need to see a doctor if the symptom doesn't subside spontaneously.