Doctors ASAP.
It could be anything. Could be that you have some food intolerance that's causing it, and if you keep eating the foods without knowing it, you could be causing yourself some very serious problems indeed. Or it could be something else. How will you know unless you get it checked.
I do doubt it was half a sinkfull of blood though, you'd be feeling that. Blood in water seems a lot, lot more than the amount it actually is.