If the rug burn gets really red, swollen, hot to the touch, very painful (not just a little sore) even when you are not touching it, oozes pus
from underneath the hardened scab, red streaks start branching out
from the burn, your lymph nodes on your neck swell up, or it starts to smell bad, then you have an infection and you need to see the doctor for some anitbiotic pills.