Why is my baby spitting up a lot and fussy?

Depends. Most babies spit up from time to time and cry when hungry, frightened, sleepy, in need of a diaper change, or simply to gain some parental attention. If crying is excessive, high-pitched, or unresponsive to soothing efforts, it may indicate a problem for which your healthcare provider should be consulted. Likewise, if spit-ups are large volume, greenish, or very forceful, seek medical attention.
Reflux or ? Could be reflux (gerd, gastroesophageal reflux disorder) which causes irritation of the esophagus (which is the tube connecting the mouth and stomach) from the refluxed food and stomach acids. That'd cause spit ups and fussiness. So might formula intolerance. But, there are other possibilities. Please have your pediatrician check your fussy little one.