That does not sound like a typical botfly, but I guess it could be some kind of reaction. In any event, I would urge you to go to a doctor who has tropical medicine training; many physicians trained in the U.S. and who have only practiced here have little if any experience with or understanding of tropical diseases.
By the way, you are not "bitten by a botfly" but the botfly egg is deposited by a mosquito or sometimes by another insect. The larva grows in the host's body until it is fairly large. There are other kinds of botflies (in the U.S. I use to call them "wolves" when I would find them in squirrels) that are deposited in the host in other ways, but in Belize I believe it is almost always by a mosquito.
The botfly larva can easily be killed by taking away its air supply -- by putting vaseline or similar on the skin where the lump is, but then you still have to extract the larva.
Belize books and publications by Lan Sluder:
Belize First Magazine (http://www.belizefirst.com
Fodor's Belize & Guatemala Guide
Belize First Guide to Mainland Belize
Adapter Kit: Belize (the first and only comprehensive guide to living, retiring, working and investing in Belize -- a best-seller on Amazon.com)
UpClose Central America
San Pedro Cool (coming soon)