Why doesn't my iphone have 3G?

I was traveling abroad for a month, (Singapore and Australia) and while I was gone, my phone would have bars from other carriers (I have AT&T) but no 3G. I thought nothing of it, considering it was a different carrier. However I arrived back into the US last night, and, despite have full bars with AT&T (my normal provider) I no longer have 3G. Any ideas as to why this is, and how I can resolve it?

All AT&T wireless networks are now LTE.
It's a system upgrade in the past month or so, the 3G, 4G is now replaced with LTE, it's basically a more smoother high data upload transition