My initial reaction is that it may be your body eliminating something that is harmful...as I'm sure you know, a woman period is the removal of the unfertalized egg from the body. Many people who zap have "die off" reactions of one sort or another. I have heard of many women who problems with their periods and were helped greatly by zapping.
Tell your doctor about your zapping should not have any effect on what he says is the cause because there is not way electricity could cause vaginal bleeding on its own. You might want to try another doctor and see if they can give you a second opinion.