That only comes down to one of the weaknesses of MBTI-test which is poor reliability, ie getting different results at different times because the result might be affected by your mindset etc. If you really want to know your type you should study the cognitive functions which interestingly INFP's and INFJ's share none. You can develop your personality, ie. cognitive functions but the fundamental type cannot change.
The thing that "changed" you from INFP to INFJ was the judging/prospecting (51% for P, that could be flipped to J only with one question answered differently).