I'm so tired~

I had been taken this allergy medince for awhile, it worked great, but I was so tired all the time. I switched to taking it at night, (the doctor's suggestion) I was hoping that would help, it didn't. So I decided to really test if it was the med. or something else before I call the doctor's office again. So I stopped taking it on Sunday, it took a few days, but then started feeling less tired. But last night I had to take it, my nose had been dripping all day, and my throat felt all closed up. Today I am tired again, how come it took a few days to leave my system, but one dose knocked me out again? I'll be calling the doctor on Monday. UGH!