Having spotting for 4 days now mid cycle. Accompanied by some mild cramping/pinching feeling lower abdomen. Why is this happening?

Difficult to say. Some women do have some breakthrough bleeding due to fluctuations in hormones. If it bothers you, you could consider birth control pills as they help to regulate your period. Sometimes extremes of weight (thin/heavy) can lead to this, as well as problems with thyroid. I would wait a few cycles, though and see if it continues. If so, then see md. Or consider the birth control pill.