I build up saliva, then when i eject it it is quite red. any ideas? doesn't happen often.

I depends...: It depends whether it is coming from the mouth, nose or lungs, and whether or not it's blood. The possible causes are many. For example, inflammation of the the gums can cause bleeding; there can be blood coming from the sinuses or nose, one might cough up blood from the lungs, or one could have a bleeding disorder which makes it very easy to start bleeding. This merits a visit to ones doctor.