Sometimes I imagine things that haven't happened yet and get really upset?

All the time, I imagine things that are upsetting (such as, a family member dying or my boyfriend cheating) and I say all the dialogue in my head and every thing seems so REAL, and it's like it's actually happening, and I start hysterically crying even though it hasn't but it seems very real. This is usually before I go to bed, not randomly during the day. What is wrong?