Could my headaches be from consuming edible cannabis?

It seems like you might have systemic hypertension—this can lead to symptoms such as pain in the hands or feet, blurred vision, or more phlegm. These are all signs of high blood pressure. You need to check your blood pressure regularly and do what your doctor says to keep it under control. Changing your lifestyle by eating well-balanced meals, getting regular exercise, and dealing with stress can make things better for you.

The problem with the shaking fingers is that I recommend seeking advice from a neurologist. Although it may not currently stand in the way of your normal activities, this could indicate an underlying problem.

It is not very likely to get intracranial high blood pressure from a single 200mg dose of doxycycline. Intracranial high blood pressure is an uncommon side effect that may lead to headaches, changes in vision, and nausea. Adequate hydration can help in its prevention. Don’t forget to follow your provider’s instructions when taking this medicine and inform them about any worries you might have. 

Headaches can arise in many ways, for example, stress, lack of sleep, or even looking at screens for too long can cause them. It's better to lie in a peaceful place, drink a lot of plain water, and abstain from excessive screen time. If the pain persists, you need to go to a doctor and get a thorough checkup done.