I would put some yeast cream on it and see if it helps. I would also dab antibiotic on any sore spots and monitor them closely. If no improvement in a day or so to sore spots, I would think infection like Amber said or depending on just how bad they looked, call the dr and ask.
The best thing for diaper rash and to also tell if it is diaper rash is, air. If you have the time, simply leave her undiapered and the air will actually clear diaper rash up pretty quickly.
If it doesn't, and it isn't some other sort of rash either from being sick...and also causes bumps, it is yeast. If it is yeast, you will see improvement through out the day using any yeast cream.
I personally would always want to treat sore spots with antibiotic, even before you talk to the dr.
Also, sometimes the diaper itself can be the culprit for a rash if your baby developes and allergy to something in it. I would do all of the above and change to a different brand temporarily.