it may be that they are trying to get you to mate with them. My GCC does this, usually when she's really excited like if i haven't seen her in hours and i take her out of her cage. She lowers the front of her body and bobs her head, with little wing flip flaps. I started just ignoring her when she does it so that she doesn't get the idea that I'm a potential mate. other things they might do is rub their butt on your hand or try regurgitating for you. just ignore it and tell them "no thank you" or "sorry, we're just friends" and they tend to move on after a while. it is more difficult if you only have one bird, so keep that in mind if it is the case.
as the others said, it is difficult to be sure without a video but that is the first thing the description made me think of. if your bird is young and just starting to get adult hormones (mine started at about 6 months) it's more likely.