There are two things to consider, the first is that the gills are filled with blood because the blood is what carries the oxygen to the rest of the body. The second is that if the fish hit the syphon and possibly banged the sensative tissue of the gill it would bleed (similar to how some peoples noses will bleed in large amounts without any real damage having been caused).
Another possible solution is that the fish could have regurgitated some stomach acid and it seeped out through the gills, not likely, but a possiblity.
I would just make sure your water quality stays good over the next few days, maybe put a little melafix and or salt if you are really concerned. But the fact that the bleeding stop as soon as it began tells me that it was probably not something serious.
good luck.