When is it safe to use a waist trainer after birth?

I'm having a hard time peeing, not sure if it had anything to do with getting the epidural, or the stitches I got. They had to empty out my bladder at the hospital because I couldn't go and im wondering is it safe to use a waist trainer? Apart from that I don't have any other problems, im still in alil pain when I try to sit up from laying down or sitting up but I know that's normal.