If your "audience" of people who usually commission you are mostly students (either in high school or college), you might get a dip in sales once they all go back to school (usually beginning in the fall) and/or have to pay for school so no one really has money for commissions. I only just opened up my commissions last month, but since most of my watchers are students, I've had no sales (and my pageviews/favorites have gone down). They might pick up again around the holidays because of winter break, or whenever else they get time off.
I have not noticed that to be an issue, but here are some possibilities to consider:
Freelance work has a tendency to fluctuate. It can be difficult to keep a consistent amount of work from week to week or month to month. If you got so busy that you stopped looking for new jobs then you may experience a lean period until you follow more leads.
If you are getting your work from one specific website, Those clients might be less active during certain months or they might be saving up for holidays or spending their money elsewhere during those months. If that's the case, then you could think about finding additional clients elsewhere.
Maybe you recently changed something in your profile or gallery that has affected sales.
If you had a run of repeat business that has ended then it might seem like a general drop in workload.
The USA just had a presidential election this week. If your clients are USA voters they may have had their minds on the election instead of commissions for the past several weeks.