Go through your list!
Babies can cry for any number of reasons. When my son was a month old there were 3 reasons. Diaper, Hungry, or Gassy. After I went through that list I usually found the problem and fixed it. Now he's almost 4 months old and that list has grown. It could be that he just wants to move a little, doesn't want to be on his back, is just plain bored, etc. Make a list of what to go over and you'll learn what you're baby is crying about.
Now we have added being restless and bored as well! hehe