Dreams get pretty lengthy when you get at least 6 hours of sleep. My hypothesis is that these short dreams are the last part of that last dream. I usually say that every episode in a REM period is the same "dream," others may break the REM period into separate episodes, calling each episode a separate dream (although the occur in quick succession). Sometimes REM periods follow one long plot, other times they split into multiple topics. Chances are that you were concentrating so hard on the topic you could remember that your mind let go of the rest (or the topics were so different that you could not remember the other topics without a cue). That's what happens to me, at least.

Regardless, soon you will remember more. Good luck, and don't worry! =)