I have a rash just inside my hips. How do I know if I need to seek medical attention?

You certainly don't have to go to the ER for that rash. However, I can't quite determine whether it's a mild bacterial folliculitis or the beginning of a yeast (candidal) infection. I am favoring the former, but the pictures aren't quite close enough for me to tell. If it's the bacterial folliculitis, try applying some topical antibiotic cream to the area, such as Neosporin cream. If you see no improvement in a few days, then you should report in to your doctor.