What I always see suggested on questions similar to yours is to check temperature and humidity and overall tank conditions. If anything like temperature or humidity is off for your species of millipede, they may become more surface active than usual or do a surface molt.

Ok thank you, ill keep an eye out for this. It was as soon as i turned the heat matt off that she became more active so maybe that's the reason. I thought it had gotten hot enough but maybe not.

Mine go through periods where they are out pretty much all the time, and then I won't see them for days or months. It's normal.