Pyoderma gangrenosum usually starts with a small, red bump on your skin, which may resemble a spider bite. Within days, this bump can develop into a large, painful open sore. The ulcers usually appear on your legs, but may develop anywhere on your body. If you have several ulcers, they may grow and merge into one larger ulcer.
When to see a doctor
Talk to your doctor if you develop unexplained blistering on your skin, an itchy rash or a sore that's slow to heal.