What Are The Uses And Side Effects Of Vyvanse?

Hi, I'm currently a student and I take vyvanse, an ADHD drug. If I take it in the morning and then work out later in the afternoon I feel fine, however if I take it in the morning and work out a couple hours later, the rest of the day I feel exhausted and terrible. Could the work out be metabolizing my vyvanse early on or does the additional increase of serotonin released from my work plus the vyvanse cause me to get tired?