Why can't I sleep after taking Mucinex DM?

My troublesome cough would've kept me up all night, so my boyfriend gave me a Mucinex-DM Extra Strength capsule. I'll tell you, I haven't coughed since, but I also can't sleep! I feel jittery and anxious, I have to get up in like 3 hours. Why am I the only person not able to sleep on this stuff??