Ironically, I saw two patients this week with the same problem. One was due to an antibiotic given at a walk-in medical clinic and the second, like you, a steroid from a dental office. The former had occurred over a year ago and looked quite severe with a grey, scarred discoloration. The latter only a month ago.
The medical name for your condition is secondary lipoatrophy. It is not clear why the fat cells dissolve after steroid injections but it is speculated that the shot affects macrophages which then sets off some type of inflammatory reaction. Interestingly, nearly all cases occur in females. Perhaps, this has to do with the increased fat in this layer. It may also be due to the different hormone profiles in the two sexes.
Intramuscular steroids should be just that: into the muscle. If the steroid is injected into the fat instead, lipoatrophy can result. For anybody reading this, make sure that when a steroid is injected, relax your buttocks. Injections should not be given into the gluteal region if the bucctoks is small. An alternative would be to split the injections into each buttock. Also, your injector, nurse or physician, should be experienced and know the injection must go into the muscle.
The potential for lipoatrophy leads many physicians to use only oral steroids when steroids are needed. However, oral steroids may trigger another problem: stomach ulceration. Da-ned if you do, and da-ned if you don't.
Usually this situation resolves itself by six months. Some authorities encourage message and you certainly should perform this 5-6 times a day.
If the problem still exists afer six months, and you are getting close to that mark, you might see a plastic surgeon. A skilled plastic surgeon could perform autologous (self to self) fat grafting. I know of a very good one here in Hampton Roads and I am sure there are many others in country.
Sculptra is used for lipatrophy caused by HIV. There may be a role for Sculptra in your situation. This lasts for 1 1/2 years or so.