If you were well booked in December, then I doubt that there was some sudden flood of new units or other market changing event that would be to blame for the slow January. Seems that this is quite common in most markets where winter is the slow season. There is always a collective "belt tightening" in January as people come out of the holidays, check their credit card statements, and promise to spend less, only to go back to their old behaviors a few months later. Plus, most people take time off in the Thanksgiving through New Years stretch because that is what we are accustomed to culturally and we already have some free days off to combine. Plus employers expect us to return to work raring to go in January, so people tend to travel less now anyway.
Either way I would recommend squirreling away money into a separate reserve account and be sure to put at least some of the profits during the better months into this as well. Even better, consider using any extra cash to pay down your principal so that you can chip away at your monthly. Every bit helps, good luck.