traumatic nightmares??

My son is 4 and developed asthma. Aside from breathing treatments and inhalers we have to give him a chewable tablet of singular before bedtime. Well he has only been on it for a week but at night he wakes up crying about things that already happened or that have not. For example he was crying last night and upset that I didn't give him a toy. This is weird he is always such a good sleeper. I mean hes 4 so when he has woken up before its just for a drink of water or to sneak into bed with me. Nothing unusual. I don't know if this is because of singular or just coincidence. Has anyone else had this problem or am I just being an overprotective mother???