Seems you have misled them with the word overload. Since you mention only one agency which would use one or a few closely spaced frequencies overload is not the problem. There is no such thing on the frequency you're receiving on no matter how strong those signals are, otherwise units in close proximity would be unable to communicate.
Now for suspect number one, you placed your antenna in a dead spot by raising it. Without getting technical above the average head, raising or lowering it or moving horizontally a foot or two should move it out of the "bubble".
Don't be confused, bubbles are frequency and position selective to a point so I'm not surprised if you only lost one station. One last thought, raising an antenna by only 15' gives fairly negligible signal increase so overload where none previously existed is out of the question.