Can a pt have a reaction to one brand of generic KCL but not another?

86y.o. female; post CHF on KCL-ER capsules without any negative reaction. Refilled prescription and was given Klor-Con ER caplets. This is the only difference in medication regimen. Now pt has red rash which began on back, spread to chest and, after several days now spread to lower legs and upper arms. Rash waxes and wans but does not disappear. Itchy only at times, especially in underarms.