Besides what Manny mentions . . .
Maybe you under-counted the carbs? 40 carbs seemed a little low to me, unless you had very small portions, so I checked a few things. For instance, 1/2 cup of Van Camp’s beans and bacon is 32g carbs and 1 oz (32 pieces) of Cheetos is 19g carbs. This is all from the nutrition labels. That’s 61 carbs for half a can of pork and beans and a hand full of cheetos.
I don’t know if you estimated your carbs or took them from the label, though. Counting carbs is a tricky art form. I am a habitual under-counter and, unless I have a label or reference for every item on my plate, I have started to bump my estimates up by one unit and have experienced better control and no lows.
One great resource if you have a PDA is the USDA Nutirent database available on-line at www.nal.usda.gov/fnic/foodcomp. Since I always have my PDA/phone with me, I have nutritional info available all the time.
The next trick is learning to accurately estimated portion sizes . . .
It never ends.