Its best to see a doctor as it could be caused by many things.
As Rumpled said, migraines
And other such conditions like Trigeminal or Occipital Neuralgia... both can be treated with a Medication as can migraines.
Its most likely NOT to be a Brain tumor.
I had occipital neuralgia...I had the twitching and spasming, my eye would go funny. I couldnt even brush my hair. It felt like millions of sharp needles sticking into my head at a certain spot. It was like an area the size of the palm of my hand. I saw a doctor, he gave me medication which helped. After a period of time, he weaned me off the medication and its never come back :)
So please dont think Brain tumor....thats the Least likely reason :)
In general, headaches on one side of the head are more indicative of a migraine than anything else.
As you cannot be diagnosed on the internet, you need to see a doctor to do tests, a history etc. Then the doctor can evaluate you for headaches or a tumor (which is unlikely!) and get you the treatment that will help.
You can also journal the triggers - it can be food, weather, perfume... so it may take a bit to figure out what caused the headache. Once you know you can take steps to avoid. For me it is perfume, low pressure systems (weather) and one of my health conditions.