Would using claritin (loratadine) d cause rapid heart rate during exercise?

Yes, any "D" drug : The "d" in claritin-d, or Mucinex (guaifenesin) - d and others, all stand for decongestant. Decongestants are medications that stimulate the adrenaline system and can cause higher blood pressure and heart rate. While not bad for short time, may cause problems if used chronically. Check with your dr. If you have high blood pressure or heart rhythm problems before using these medications.