I have itching and a few bumps along my underwear line. I'm heavyset and sweat a lot. I tested for STIs, all negative. What could help this rash?

Likely yeast. Women can get a yeast that is more like the yeast men get with jock itch. This is called tinea. An antifungal powder with nystatin or mycostatin should help. Once it clears using an antipersperant in this area (avoid ones with fragrance) will help keep the area dry. Taking the underwear off near bedtime to allow cool air over the area helps too. Even better a cool fan.