You may find this is a form of night terror.
If you do suffer from these you will find they normally occur whilst going to sleep, or whilst waking. Quite often they vary from people to people, and also seem to play on your subconscious, for example I have a fear of spieders, and if I have an attack normally I experience mild hullicinations involving them.

However it is always hard to determine whether feelings experinced are hallucinations r not. It sometimes makes you think you may be losing it, it definitely did with me.

Therefore as with most things the best option is to schedule an appointment with your doctor or physician , whereby you can discuss possible side affects of your medication, how to work round them
, and also in the worst case, talk about testing for possible sleep disorders, and in the worst case any other medical problems they think it could be.

Take note though if it is the last option(i hope not) be prepared to wait a while, it took approx 6 months to confirm that I had narcolepsy from when the doc first announced his suspicions.
Quite often after ejaculation the natural response for the body is to relax, therefore especially if lying down you may start to go to sleep. As regards