I've heard that people taking dex (mostly in Adderall form) can find times when the meds are wearing down that they feel either angry or sad. It may be the body coming down after having the flood of chemicals in the brain. Or perhaps an adjustment in timing or dosage could be needed.
If you are taking SR version of dex then timing is important. The med lasts a very short time so try to stagger them to flow from one to the other better. Or you could try the spansules as I do - then the most you should ever need to take is twice daily. Mine works optimum about 6 hours so if I have a long day I take my first when I wake up - then time the second for 6 hours later. It works very well and I honestly never feel sad anymore. Sometimes quiet and reserved but mostly very very happy!