The anconeus muscle is a synergist of the triceps brachii and can give you elbow pain if you overwork it.
The amount of repetitive extensions of your elbow, that are too much, are an individual matter and depend on factors like training status, diet and the state of your nervous system, etc.
In general, the same activities that can overwork the triceps are relevant.
In sports, this may be exercises like the bench press, dips or swimming.
However, activities which do not seem damaging at the time may be too stressful for a particular individual.
This could be something like pressing down a piece of wood on a bench to stabilize it, so that your partner can cut it, or holding a heavy drill in order to place a bolt in rock or dense wood.
Finding the anconeus is not too difficult.
Use your fingers for the massage. A massage ball might already be too big and thus will not get on the muscle properly, but your fingers will be just fine.