You are not the first person that has suffered from this. Its kind of annoying when you have a nice custom avatar, only to have it removed later on.
Technically, you should never have been able to upload a custom avatar. As a Junior Member, your avatar permission is still restricted to the stock avatars hosted by CAF. But what seems to happen from time to time is that the system has a glitch, possibly because it was hacked, which allows members to have graphics permissions they ordinarily wouldn't have.
When the glitch is patched, members still can enjoy treats they normally aren't entitled to at their current membership "rank." But this only lasts for a time. After awhile, the forum software's automatic error correction program will wander across this situation and "fix" it. In your case, the software detected that you had custom avatar before you were entitled to one, so it removed it from public view.
Here is a list of the different levels of avatar permissions
You will qualify for a custom avatar once you become a Regular Member. That "rank" will be given to you automatically once you have been a member for six months AND have made at least 500 posts.
Here is a list of the different membership levels and how to qualify for them.
Again, I am sorry this happened to you. I imagine it must be frustrating.