Spotting in between periods? Could my change in exercise and intensity be the reason for it?

Hi there, A bit of a background: I'm 20 years old, exercise every day, in pretty good shape (5'10" about 135 pounds), and I eat a clean diet.

My period started on July 12th, and probably went until the 17th or a few days after that, normal flow and everything. Today, I woke up and realized I was spotting which has never happened to me between periods before.

I recently change my workout routine. I used to just go for long bike rides, walk on an incline, or do a long cycling class at the gym. I switched to high-intensity interval running and started lifting weights about a week ago. Could my change in exercise and intensity be the reason for my spotting? Also just to include, I don't take birth control or any medication and haven't been sexually active. The only thing I could think of is that it's stress from being in the process of moving or my change in exercise.