Why does my daughter wake up in the middle of the night?

My daughter will be 2 in september. Every night for the past 2 weeks she has been waking up in the middle of the night and start crying and screaming for a few minutes and fall back asleep. She has never done this before. Since she was 2 weeks old she has slept through the night. Besides that she has been waking up in the morning a few hours before her normal time. My mother suggested that mabe she is having nightmares and that is what makes her wake up everynight. I believed it for the first 2 nights but after that i find it kind of too much. Its been two weeks and its becoming a problem because i live in a small house and my mother lives with me. She has a hard time sleeping and any noise wakes her up so i know she is being disturbed. Does anyone have any idea what it could be that is making her do this? Is there any way i can try and prevent this? I have tried everything i can think of to help her sleep better (bath before bed, no bath before bed, pajamas on, pajamas off.... ) Can anyone suggest anything? Thank you ahead of time