Your theory is most probably spot-on. Read on...
This might help, from Diana Walstad:
The link has some good graphs....
Nitrogen Removal in the Planted Aquarium
Nitrogenous compounds, particularly ammonia and nitrite, are probably the most common pollutants of aquariums. Both ammonia and nitrite are extremely toxic to fish. Aquarium hobbyists depend on biological filtration to convert these toxic compounds into non-toxic nitrates. However, plants and soil bacteria can also remove nitrogen from aquariums.
Aquatic Plants Prefer Ammonium over Nitrates
All plants can use either ammonia/ammonium, nitrite, or nitrate as their N (nitrogen) source. Based on N-uptake studies in terrestrial plants, many people assumed that aquatic plants, like terrestrial plants, mainly take up nitrates. Actual experimental studies with aquatic plants suggest otherwise.
Scientists from all over the world have studied N uptake in aquatic plants under a variety of experimental conditions. From the published studies, I found 29 species of aquatic plants that preferred ammonia/ammonium to nitrate. Only 4 species were found to prefer nitrates.
Moreover, the extent of this preference is considerable. For example, the aquatic liverwort Jungermannia vulcanicoloa was found to take up ammonium 15 times faster than nitrates . The duckweed Lemna gibba removed 50% of the ammonium in a nutrient solution within 5 hours, even though the solution contained over a hundred times more nitrates than ammonium .
Elodea nuttallii, placed in an equal mixture of ammonium and nitrates, removed 75% of the ammonium within 16 hours while leaving the nitrates virtually untouched. Only when the ammonium was gone, did the plants begin to take up nitrates.
Likewise, when Ferguson  grew the giant duckweed Spirodela oligorrhiza in media containing a mixture of ammonium and nitrate, the ammonium was rapidly taken up whereas the nitrates were not. Because he grew the plants under sterile conditions, he showed that the ammonium removal could not have been due to the bacterial process of nitrification.
Also, Ferguson showed that plants grew rapidly suggesting that the ammonium uptake probably accompanied the normal growth process and increased plant material (the N concentration in aquatic plants ranges from 0.6 to 4.3% of their dry weight ).
Nitrite Uptake by Plants
Although plants can use nitrite as an N source, the pertinent question for hobbyists is- do aquatic plants remove the toxic nitrite before the non-toxic nitrate? I could not find enough studies in the scientific literature to state conclusively that they do. However, the chemical reduction of nitrites to ammonium requires less of the plantís energy than the chemical reduction of nitrates to ammonium
. Thus, when Ferguson  grew the duckweed Spirodela oligorrhiza in media containing both nitrate and nitrite, he showed that it preferred nitrite.
Stop dosing N until you get '0' nitrIte readings for a day or two....