I don't know the exact reason, but I can offer some pointers.
Your bed is an area which smells very strongly of you, so it's the place where she feels closest to you. Peeing on the bed is often a sign of insecurity. You say nothing has changed, but could there be something which you're missing/haven't thought about? Maybe a guest she doesn't like? Or a local cat (if she's not going out, she might see one through the window).
Your next problem is how to change her behaviour. I'd suggest giving her a bed with an unwashed item of your clothing put into it. This will have the same scent as your bed. The smellier the item, the better (not that you smell, but if you wear scent at night, give her something with that on, even better, try an unwashed nightie). If she normally sleeps in your bedroom, put this bed in the bedroom, beside your bed and encourage her to sleep there.
Another consideration: can she easily get to the litter tray from your bedroom and is the litter tray kept scrupulously clean? Most cats are very fussy about using soiled trays, so she'll choose your bed instead.
Hope this helps.