Prescribed allegra (fexofenadine) m once daily for 2 months and levocetirizine 5mg at night if necessary but feel allegra (fexofenadine) is causing upset stomach and fast pounding ?

Then stop it: and check with your doctor, arrhythmia isn't known to be caused by fexofenadine, that was a known side effect of older antihistamines that were withdrawn, but given the symptoms you are suffering and you suspect very much fexofenadine (allegra) to be the cause, you can safely stop and ask your doctor for an alternative, and there are others, continue the levocetirizine, till you see your doctor