From December to April, consumer prices in the United States rose a scant 0.1 percent a month. Even Reagan administration economists cautioned the good news could not last, and with the May report on the consumer price index, the lovely bubble burst.

The abrupt drop in inflation to an annual rate of only 1.5 percent early this year was primarily the result of some of the volatile prices in the index either slowing sharply or, in the case of energy, actually declining.

Data Resources Inc., an economic consulting firm, recently compared the changes in these volatile components--food, energy and homeownership costs--during the two years ended in December 1980, when inflation was at its worst, and in the four months ended in April. Together, these components account for nearly three-fifths of the total index.

During 1979 and 1980, the CPI rose at an annual rate of 12.9 percent. Food and beverage prices rose at a 10 percent rate, while homeownership costs went up at an 18.1 percent rate and energy at a 27.4 percent rate. The remaining two-fifths of the CPI climbed at only an 8 percent rate over the two years. (See Table.)

In the four months ended in April, on the other hand, energy prices dropped at a 13.1 percent rate, homeownership costs went up at a 2 percent rate and food at a 4 percent rate. The remainder of the index rose at a 4 percent rate.

The overall slowdown in inflation from those two years to that four-month period was 11.4 percentage points. Given their relative importance in the index, and the degree to which inflation abated, energy accounted for 4.4 percentage points of the slowdown and homeownership for 4.2 percentage points. Food contributed 1.1 percentage points and all other items 1.7 percentage points.

In May, all three volatile sectors bounced back. Gasoline prices did the most damage followed by higher homeownership costs. Some analysts questioned whether the 1.8 percent rise in home ownership costs, much of it because of a 2.6 percent jump in new home prices, accurately reflected the true movement of home selling prices when the housing market is so severely depressed by the high level of mortgage interest rates.

Food prices went up 0.8 percent, but increases in that sector are expected to moderate later in the summer. Indeed, the surge in gasoline prices probably will be over within another month or so, with smaller rises later in the year, according to oil industry experts.

Even while the overall index was doing so well earlier this year, DRI's Sara Johnson noted that "high inflation persisted in some markets protected from price competiton by government subsidies, regulation, or import restrictions.

"Residential fees for electricity and natural gas surged at a 15.7 percent rate . . . and double digit inflation rates were recorded for personal and educational services 12.6 percent , medical care services 11.4 percent , tobacco products 11.4 percent , and medical care commodities such as prescription drugs 10.6 percent ," she noted.