Can an infected tooth cause a swollen mastoid lymph node? i have a smooth hard lump right behind my ear on the same side no pain and doesn't move.

Not likely: Mostly we see swollen nodes in the neck or under the jaw with dental infections. You do not say if you do have any dental symptoms. I personally have never seen mastoid area nodes swollen due to a dental infection. This could be a abnormal lump that should be checked out and has no relationship to a bad tooth.

Is possible but rare: I have seen a few patients with a mastoid swelling that resolved when they had an infected upper wisdom tooth removed. You should still be seen by a physician in case something else is going on.

Possibly: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.