Well. Good news. You are experiencing an ocular migraine
-- it's not a brain tumor...
A typical migraine headache starts with shimmering lights, often times they surround a blurry area or have dots or jaggedly lines associated with them. They tend to progressively increase in intensity and sometimes march across the visual field causing difficulty with reading. Many times this is then accompanied by nausea, irritability, sensitivity to bright lights and/or loud noises. After the onset of the lights (called scintillating scotomas), the headache typically starts and the light show tends to progressively go away.
Many people can have this migraine phenomenon without the headache; it is called an acephalgic migraine. Some people even start having these late in life, or may have had a few much earlier in life that behaved differently and haven't had any for decades and then begin to have them; this is not uncommon. A family history of migraines is often present as well.
This is nothing to worry about. It is not a sign of a more serious underlying condition, brain tumor or anything like that. If the episodes become so frequent that they are bothersome there are medicines that can be used to decrease their frequency or stop an episode once it has started.
I, personally, have been suffering from this condition for almost 30 years. I almost never got the headaches after the visual effects and, hopefully, this will continue to be your experience.
Does this make sense to you?