Toddler suddenly waking through the night?

For the past 2 months my 16 month has been waking screaming at 3/4am (sometimes I'm lucky and it'll be 5!!). Since she was 6 weeks old she has slept through the night and up until recently has slept 12 hours a night with a short nap in the day. I have tried earlier bedtime, later bedtime, she has blackout curtains in her room so it's not the light mornings. Any advice will be great pleeeeeease!!? Xx