There is a such condition as a malformation of the tear duct that can prevent tears when crying. Also it can be a sign of dehydration. You should check with a doctor. It is common in babies as a sign of dehydration but not so common in adults. If there is a malformation of the tear ducts, there may be a way to correct it.
sounds like you are not realy cryind you are trying to force a cry because if you cry there has to be tears..