gender confused.. again. :|

hi guys~
ive recently bought a new betta,, supposidely female.... but i keep getting this hunch that its actually a male. for one, shes MUCH bigger than anyone ive kept before..even my full grown males arent as big as her and her body doesnt seem too female-like to me. its not rounded like the others. ive checked to see if shes got an ovipostor, and shes got one but its pretty small..also shes pretty docile, and she doesnt flare at anyone..
anyways, im at a loss... and i would appreciate anyone's input!