They go up and down. If PPC is shrinking, new entrants will come and bid the price up. May be some advertisers don't like your site conversion and and they have pulled ads from your site.
In my cases, CPC stay the same, only lower in CTR. But I agree the earning is pretty bad since the first Jan. Wondering this year could be a bad year because of the bad start... Well, just think positively and do extra work to boost it up, especially the traffic.