Please start birth control or have your partner use a condom. The morning after pill is not intended to be used as contraception on a regular basis. The morning after pill is basically a high dose of the bcp and it can really mess your hormones up when taken routinely. Yes it could be that it affects your hormones and causes a false positive. Plan b should only be taken in extreme cases as in rape, incest, or for a random issue such as a condom breaking. Please stop using it routinely it could end up causing you to have long term hormonal issues.
That pill is not meant to be used more than 2 times in a year (at least that's what doctors have told me). It can really mess up a woman's body so yes, it could be the chemicals but I would definitely seek professional help for this one. Good luck!
If you have taken the morning-after pill three times in four months, anything is possible with your body and your cycle. It is not meant to be used that often. Please see your doctor and see if you really miscarried yesterday and what your uterus looks like now. And get on a birth-control regime that does not require continual use of the emergency contraceptive. It's not recommended to use it that often.