I've noticed that I do get very graphic and unpleasant dreams while taking SSRIs in particular. I do find these meds affect the quality of sleep I get too, i.e. it isn't always refreshing sleep. I don't really know how the cause and effect works, but I suppose it is because SSRIs don't actually release serotonin, they prevent the body re-absorbing what is already there, so as to make more of it available for transmitting messages across the synapses. I guess that having more available serotonin means more messages can pass across the synapses, even when we don't want them too - like when we're asleep and it is this increased capacity for transmitting messages that causes the dreaming.
My usual nightmares are not being able to find a toilet, or finding every toilet that I visit is filthy. Those don't take much interpretation. I have dreams about my teeth falling out, again that is quite a commonly occuring nightmare. The other night I dreamt my dog came home without her tail and that it was sent to me by post three days later. There are others too, these are far more graphic and they leave me struggling to re-orientate myself when I wake.