When should she take pregnancy test?

i had sex(anal) wid my gf on 19 july it was second day of her period , we taught some semen went near her vagina so to be sure she took i pill on 20th july.she didnt had ny bleeding after pill ,but on 11 aug she had some blood jus drops ,n on 15 aug she had brown blood again that 2 few drops before her normal period (20 aug) wid some nausea ? is this normal.!? 
n when shuld she take PT?,if she miss her period on 20 aug wen normal period r excepted?
plz ans :(