Most probably you sing from your throat. Ganun din experience ko eh. I tried singing from the diaphragm. Nakakapagod siya, parang may hangin sa tiyan, pero at least, makakakanta ka parin.
EDIT:
Mukhang medyo malabo yun ah. Umm, kasi, when you sing from the throat, parang mahina lang yung level nung voice mo. Kapag from the diaphragm, parang pasigaw yung boses mo na hindi.
Still, parang medyo malabo yun...