IT'S BEEN A terrible few weeks. Just over a fortnight ago everything seemed to be going along just fine. The president had won a landslide election, and in his post-victory press conference he assured everybody that all that was needed to keep the country humming was to "continue what we've been doing" -- and maybe tidy up government operations to make things run "more efficiently and economically." Now, all of a sudden, the happy sounds emanating from the White House have turned ominous.

There is talk of cutting back services for veterans, pensions for civilian and military retirees and subsidies for farmers. Aid to education and grants to states and cities are back on the chopping block. The idea of making most companies pay taxes again is being floated.

Even the economy, which was supposed to be launched into a "new era" of robust growth, seems to have gone flat. It turns out that it only grew by 1.9 percent in the third quarter of this year, much less than the administration initially estimated, and the outlook for this quarter isn't bright. A less sunny forecast, of course, means further additions to budget deficit estimates, which had already been growing by leaps and bounds -- over the last couple of weeks. How did things get so bad so fast?

White House spokesman Marlin Fitzwater denies that the discussions reflect a change in administration budget policy. With respect to bringing down the deficit, Mr. Fitzwater remarked last week, "I urge you not to buy the idea that growth would do it alone; I don't think anyone ever said that." That's funny. We thought someone had. Someone like the president.

Take his first debate with Walter Mondale, for example. (You remember Walter Mondale, the one who kept talking drearily about how budget deficits posed a serious threat to the country's future. You don't?) In that debate, President Reagan said that the only plan he had to deal with the deficit was "the economic recovery program that we presented when we took office in 1981." But that plan -- big tax cuts, a buildup in military spending and substantial cuts in social programs -- has already been put into law. Congress rejected a few social program cuts, but it made up for them by approving somewhat smaller defense budgets. True, the president also mentioned during the debate his wish to reduce the "rate of increase in government spending." But that didn't sound like anything very serious in the way of cuts.

Two prospective explanations for the sudden change in the tenor of the White House debate are being voiced: the "sham" argument and the "oblivious" argument. The first holds that the president knew all along that stern measures would be required but decided that it would be unwise to alarm the public before the election. The second holds that the president, despite his nearly unlimited access to the best economic analyses and data, didn't know what was going on. Neither explanation can be one the administration wants to press.

There is, of course, also the treachery explanation. This theory, propounded by die-hard supply-siders, holds that the numbers, or many of them anyway, are being cooked by administration figures, such as David Stockman, for their own nefarious political purposes. Is there a fourth explanation? Does the administration have anything to say on this that is better than the cynical explanations that so readily come to mind?