Ingrown armpit hair has doubled in size across my right armpit at the top. it's red and started off about 2cm, it's now about 5cm wide and is burning.

Abscess: What you're describing sounds like an abscess in the armpit. Shaving, certain deodorants, and other factors can cause this to happen. At this point you should seek treatment, which will likely include an incision and drainage procedure. An urgent care and many family physicians can easily handle this. You may be prescribed antibiotics as well. Until then, a warm compress may help it drain