I have something like that myself. It’s on my inner arm near the elbow. Just showed up one day red and hard. I thought it was a spider bite. It took several weeks to go away and it still has a slight redness and a small, hard lump. It never drained or seemed to be pus filled. It was sort of like a sebaceous cyst, but irregular.
You know I’m not a physician and can’t make a diagnosis of anything. I can give you my best guess based on your description, but that’s about it. If it’s something that hurts or is being rubbed by your clothes or gets in the way of your PE, then you probably should have a doctor look at it. Good luck.