Is the area smelly, moist, red, irritated & getting raw? If not it is probably not a hot spot, it is probably her anal glands that needs to be expressed. Your vet can do that for about 8 bucks & they can show you how to deal w/ it in the future, you can also help prevent this problem by her diet, adding a little more fiber to her food (they make diets more high in fiber, or you can add a tiny bit of canned pumkin etc depending on her size).
If it is a true hot sopt & boy you will know it it is a hot, smelly, moist, raw spot that is un-bearable for dogs (it is ussually caused by excess moisture, not drying properly or getting wet too often, or im-proper grooming). In order to cure a hot spot dry air is the most important step (ussually be shaving the area plus 1-2 inches around the infected area) ussually an anti-histomin is given (benadryl) if bad then a cortison cream ir injection. Also in sever cases antibioptics must be given.
Sulfodex or sulfoden makes a hot spot medical application (I know it can be found at wal-mart & most pet stores). It works if you catch it before it gets bad.