Could be explained by hypnogogic imagery, although then you would have to explain that and how it's integrated with psychedelic effects.
Don't take such high dosages IMO, about 3 mg should be absolute max. I get such potent effects with about 1 mg or less (often just 250 ug) and wouldn't want to take much more. Yes it can be sedating somehow, but melatonin plays a role in your day and night rhythm and is a terrible candidate for using at any time other than the optimal moment to boost that rhythm or before it. For any other application like inducing sleep in general you would want to choose something else otherwise you are promoting a detrimental shift in your rhythm (at least, for most people with more trouble with late nights than early nights).
After tripping, melatonin is a poor choice too because again it wouldn't really aid much in counteracting the effects which keep you from sleeping (discounting complex interactions) - take an actual sedative if you must take anything. A serotonergic antagonist would work too but would be overkill and not the most responsible choice. You don't really want to fuck with your serotonergic system anyway unless you had to, just after you have done profound things to it.
Read the book Hallucinations by Oliver Sacks to discover how there are countless types of hallucinations which don't have to do with psychedelics and plenty of which don't even involve disease or dysfunction.