Why is my horse so aggressive all of a sudden?

I own a four year old (almost five) American Quarter Horse gelding. Lately, my horse has been fairly well-behaved with a good attitude. I could ride him easily and do all kinds of groundwork, such as lunging, with him. Now, within the past week, he seems to have turned into a monster. Whenever I touch him, he maliciously tries to bite me. I try to lunge him, and he charges the tassle on the end of the whip, then me if I try to chase him away. Now, I know you might say to show dominance, but every time I try to do that, the problem intensifies. It's dangerous. He has exhibited this behavior before, but lately he had seemed fine. However, he has seemed constipated the past few days, and I did as the vet directed; I gave him two grams of phenylbutanol, walked him, and kept him watered and fed with roughage. He seemed to be in pain, but he's had these problems before, and this time does not seem like an adequate excuse for such awful behavior. How do I get rid of this problem? How do I boost my confidence?