Coughed up blood

[brimmond2]

Took my 4yo son to the dr 2wks ago, bad chesty cough through the night, waking early hours, coughing, being sick etc. She examined him, said chest was clear so we left... Friday night - he woke 1.30 am coughing badly, lots of dark mucous came up. Knew it was blood by appearance & smell (sorry tmi). Called NHS 24, took him straight in. Saw 1st dr who said def blood after examining tissues we,d taken in (told to bring them). No temperature just this awful cough. Examined again by senior dr, throat swabs taken etc... Should mention too that my son had tonsils out in November 2015. She said he may have ruptured something due to all the coughing OR could be collection of old blood after his op just coming up now. Right enough it was very dark in colour, brown, not fresh red. Throat swab results back from hospital yesterday say no bacterial inf but positive for Rhinovirus.
Would this virus cause blood to come up? Just concerned it happens again, wasn't a nice experience & would hate to think there is something else wrong.