I found a small tick (nymph?) just above my left posterior thigh 3 nights ago, but did not immediately remove the tick. now i have a slightly swollen rash that is spreading red tendrils. i started applying neosporin today. now what?

Cellulitis: Any break in the skin can introduce bacteria and cause a skin infection (called cellulitis). Initially the surrounding area may get red and warm and then you may see streaks spreading from the infected area. Since this is currently your situation, it is vital that you see a Dr and get antibiotics. On Telemedicine if not in person! The infection can continue to spread into the leg and blood stream.