What Causes Painful Lump Under The Arm?

I have had a pea-sized lump on my right underarm since yesterday. It is painful to touch and the area around it hurts if I lift my arm. It is not visible and the site is not red. There is no pus visible. The pain is dull. I ve also had a constant, piercing headache in the temple and behind the eye on my left. What do you the cause is? Should I see my gp asap or continue to monitor for now?