Is a minor headache normal when getting early back to doing strenuous abs exercises after not doing it for a month or so?

Headache with abs.: This could be normal. Depending on what exercises you're doing for your abdominal muscles, such activity can put strain on the neck muscles, which can be reflected in a headache. As well, holding our breath while we perform the active part of a rep can lead to a headache as well. Should these headaches persist, though, see a doctor to be sure that their cause is such.