First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes. The symptoms you describe are a common concern when tapering off of Decadron with brain tumors.
Increased intracranial pressure from edema related to your tumor can cause headaches. These headaches are worse with coughing, straining (like using the bathroom) and bending over. These headaches are also often associated with early morning nausea/vomiting. When Decadron is started these headaches often get better as the edema (swelling) does down, or after surgery/ radiation. When the headache returns, the concern is if the tumor is back or if it is precipitating migraine headaches.
Migraine headaches can be precipitated by structural lesions(such as tumors), but care must be taken to rule out other causes. The MRI (as you mentioned) can evaluate for edema, and evaluate for additional lesions. An EEG (electroencephalogram or "brain wave test") should also be done to make sure seizures are not coming from the tumor causing your headaches (could also cause visual "light shows" in your case). Decadron can also calm migraine headaches and when removed they can get worse (however, due to the side effects of decadron, it is not recommend to use steroids of migraines long term).
The Decadron is a steroid that interacts with the hormonal circuitry of your body. While on the Decadron, your body does not make its natural steroids. So when you stop the Decadron it takes a while for your body to catch back up. That is why you should never abruptly stop Decadron, and a long taper is often needed. The lack of your own steroids can make your feel fatigued, etc.
From the symptoms/history you describe I would recommend that you start a migraine preventative medication such as elavil 10mg-25mg at night, and get the EEG previously mentioned. If you continue to have the peri-menstrual migraines a long acting triptan (such as Frova or Amerge) can be taken daily for the 4-5 days your are at risk for developing a headache. Your neurologist will be able come up with a tailored headache plan for you. I hope this has been helpful.