I tried to take its picture but it is too fast and keep on flying around. The sunflower haven't bloom yet so there is not much for it stay for long. On its two fat legs, it has some hairy yellow stuff. I am not sure if those are pollen or it's just its hair. Overall, its body is pretty yellow. Could it be a bumble bee?
Hard to say without a photo
Hi. I was waiting to see if someone else would jump in here with advice on how you could ID your bee, but I guess they're as stumped as I am. It's impossible to say what kind of insect you're looking at without a lot more info and, preferably, a photo of it.
But maybe you can find something like it at http://bugguide.net/
That's not an easy site to use (at least, I don't find it all that easy), but it's got a very extensive set of insect photos. You can start out in a couple of ways: 1) look at the black-and-white drawings at the left side of the page and click on the one that looks like your bee, or 2) enter some search terms that describe the bee you're seeing.; it sounds like it could be a lot of things, but, since it's fat and yellow, I'd start with "bee fly" and "bee."
Whatever method you use, be sure to scroll down toward the bottom of each page. That's where the photos are that will help you figure out what your insect is.
You can also try http://saltmeadow.com/pollinators/ Not very many photos there, but it's quick and maybe your bee is among them.