C.G.
Since you say that it is "pimple like" it makes me think yeast. Yeast rashes tend to be bright red, raised, and patchy. Nystatin is very commonly prescribed but is actually not great for yeast- about 60% of candida (most common yeast) is resistant to it. You may have to get another med.
I used (and recommend to my patients) gentian violet for oral thrush in my baby. I don't think that can be used on the skin (I could be wrong) but I would recommend looking into some alternative remedies- in my experience they are much better for yeast. There is a creme recommended by Dr William Sears that is supposed to work well- you will have to look on his website though as I am not sure what it is exactly.
I would contact the doc ASAP as well- there's no need for your baby to be in pain. For comfort you could buy some plain, unsweetened yogurt and put on her bottom- it will help with symptoms and can curtail the yeast. Also avoid sweets and carbs while she is broken out- they contribute to yeast growth.
Keep her out of a diaper as much as possible and exposed to air. Try to avoid baby wipes as they irritate- cleaning her off in the sink with water (no soap) and a washcloth is best. If you have ever cloth diapered it may be helpful to switch to them so the irritants from the disposables are not affecting her. Either way make sure to change frequently to avoid her being in wetness. A little Tylenol may be helpful for her pain. Hope she feels better soon.