When you sleep with someone, your energies merge and they remained merged for up to 72 hours afterwards - if you're clairvoyant sometimes you can actually see this - you carry some of their energy with you and they carry some of yours. It brings about a feeling of being close to that person but also means you can take on their 'stuff'.
If your ex was dealing in negative energies, it's likely you took some of those on. You can shed them just as quickly by intending to and by visualizing them gone.
If they are pissed will they do anything about it? How can I tell? What CAN they do to me- while awake, while dreaming, while projecting (if I ever figure out how....)
What negative entities can do to you all depends on your beliefs. If you believe you can be victimized by negative forces, it's possible for you. It's also possible to remain unaffected by negative forces...
When you take into account that negative forces appear to be very powerful but lack a source of energy (as they do not work with Source/God energy), it's easier to deal with them. I explain this in my blog post about overcoming a fear of the spirit world...
Negative entities are only as powerful as you allow them to be because they draw their energy from you when they're scaring you and victimizing you.
But you don't have to buy into them. They only harm you if you believe they can.