Yellow sac spider bite won't go away

I was on my computer late at night and my back started to itch really bad all of sudden. So I take a look in a mirror and it looked like four big mosquito bites(2~4 inches apart), all red swollen itchy. I didn't know what it was at the time, I thought it was just mosquito bite, so I just left it alone. And couple days later, the 2 out of 4 bitten areas has turn slightly purple, but swelling has gone down, so I left it alone again. But I was starting to think it wasn't mosquito so I was on the look out for any bugs, and just the other day I found yellow sac spider in my room. So Im pretty sure that was the culprit, but it's been 2 months and the 2 bitten areas are still kinda itchy and reddish(I think my skin already undergone necrosis and in process of healing?). My questions are: are there anything I can do to treat it? I've only heard people say, "It will heal over time naturally." Are there any way to expedite this process of healing?