help!

ok mommas, I potty trained my almost 2 yo earlier this month. she did so good and was in panties all the time at home except for sleep. even then she would tell you she needed to go. more often than not she had dry diapers. almost 3 weeks in and she has completely regressed in the last 3 days. there haven't been any big changes to her normal routine. it's almost as if she was never potty trained. she will pre in her panties right in front of me. any guesses as to why we have gone completely backwards?? thanks!