Potty training regression advice needed

SRose27

my 3.5 year old DD was doing great on the potty since she was 2 and a few months ago she stopped going #2 in the potty.  She was holding it in and wound up having to give her Miralax on a regular basis.  Now she will only go in her overnight diaper or in her underwear! I am getting very frustrated and not sure what to do.  She knows she is to go on the toilet, but doesn't.  You ask her if she has to go, she says no, and then 2 minutes later she does it in her underwear or bathing suit.  Any suggestions on what to do about this? TIA!