If you smoke marijuana, this would be the cause.
If you don't smoke marijuana, then you do dream but you don't remember the dreams. You need to train yourself to remember them. If you wake up at night then go over what you glimpsed in your dream, try to remember as many details as possible. Keep a dream journal.
And before you go to bed, start an affirmation. Begin this on the Pisces New Moon this Sunday before the VoC, or the Taurus Moon next Wed/Thurs, or the Cancer Moon on the 18. As you lie in bed ready for sleep, breath in indigo or blue energy (silver if you are more experienced, but avoid silver if you are new as it might be gray instead which malefic). On the exhale affirm something like "I am having vivid dreams and remembering all the details, in beneficial ways for me."