I've been getting what feels like a mild headache every day for the past month. what could it be?

Headache: It is not possible to give you a relevant differential diagnosis since you have not provided any info.. Do you have any medical diagnosis or on any meds? are you a smoker? do you consume caffeine? what time of day do you get the HA? Do you stare at a computer screen all day? You should have a physical exam and have your BP and blood sugar checked along with full evaluation by your doctor?

Tension headach: More history Time of day am , pm What areas are affected -- forehead , around eyes, back of neck. both left and right side ? Have you taken any over the counter medication (OTC's) Tylenol, Advil , Aleve (naproxen) ? If so did it relive the headache ? How is you vision ? Do you need glasses or or change current glasses proscription. Eye strain can cause tension headache. See your doctor

Headaches: New headaches that occur early in the morning and wake the person up from sleep or cause vomiting are serious and need immediate evaluation. Other types of headaches should be methodically evaluated by a physician to reach an accurate diagnosis

Headache: Not to be facetious, but it's probably a headache (HA). There literally hundreds of causes of headache. A 'mild' HA without other neurological symptoms is unlikely to be of serious consequence. However, if you are concerned you should see your Primary Care Provider.