Are you referring to the Serbian Cross?

The symbol is derived from the Byzantine Palaiologos dynasty's imperial banner, where variants of this tetragrammic cross have been in use even before the Palaiologai, who ruled from 1261-1453 CE. Here is an example from the reign of Romanos IV Diogenes around 1070 CE. There is debate on whether or not this Byzantine version represents firesteels or the letter "β". The "С" (or S in the Latin alphabet) shape in Serbian variants is also based off the Byzantine version, and the legend goes that St. Sava (1174-1236), who founded the Serbian Orthodox Church, created the "C" version to represent the motto "Само слога Србина спасава", or "only unity saves the Serbs" (which I'm sure you know the meaning to).

Some historians propose that as early as the reign of Stephen Dušan, it was adopted as a Serbian national symbol after he had proclaimed himself "Emperor of the Serbs and Greeks", roughly around 1346 CE, though the usage of the cross in Serbia predates its use as a national symbol. The Byzantine influence likely came from the fact that he had spent much of his youth in exile in Constantinople, spoke Greek, and was acquainted with Byzantine customs. From his title and subsequent military ventures in Thrace and Macedonia, it has been speculated that Dušan wanted to created a united Orthodox Greco-Serbian empire and replace Byzantium in that sense.

Ultimately though, it has its origins in Byzantine heraldry, Byzantium having had a huge cultural, religious, and political influence on Serbia since the Slavic migrations to the Balkans in the 6th and 7th centuries. The theory is interesting, though it probably isn't very plausible.