I guess it must be worrisome if this has never happened to you before, but any guy will tell you that sometimes for whatever reason, erections and ejaculations will vary both in arrival time, speed of release, and quality. This even sometimes happens when a guy is with a partner, let alone just himself. Different moods, physical states, distraction levels, emotional statuses and rest levels can all affect every part of our lives, and erections are part of life like everything else. Sometimes just a random thought on an unpleasant or arresting topic will make an erection go away, and sometimes a random thought will make it arrive even when you don't really want one. It's not a matter for a doctor or a need to talk to your parents, this happens sometimes, no big deal.
Update: I've been reading up on cold medications linking to DE and such problems, and it seems that some people have experienced this problem as well...