Oh this is a tricky one and probably pointless me posting as I can't give you any answers but........ what is the routine around the time when she wees on the bed ?
For example, could she be getting excited about being fed or taken for a walk or could she be getting anxious about her mum leaving for work for the day ?? or as someone suggested could she be busting and hold on no longer ?
I would try and review the routine and look for a possible trigger ?
Best wishes with it, these things can be frustrating !
Harry (tri) and Digby (blen).