Child Gets Frequent Constipation, Takes Pedia Lax Every Month, Low Appetite. Cause?

I took my 4 yr old to our doctor to get a cough checked out that she has had for almost 2 wks (just like she is constantly clearing her throat ). Doc said everything else looked fine and said it was allergies, gave her zyrtek 1/2 tsp once daily. 3 days after starting the med she had a loss of appetite , was jittery and slept for the better part of 2 days. Monday I called doc and she said to stop the med and call back in 4 days. That night she started running a fever 100.4, had nausea (vomited once) and very sleepy. Today (tues) she has had a fever with ice cold feet and a little shaky when the fever comes on. She is also constipated quite often, has been for about a year now and I just give her pedia-lax usually once a month. Do you have any advice as to if this sounds like a cold or something more serious? Thank you