Implantation Bleeding or something else?

Brandi

18 years old. Very regular periods. Started spotting on Cycle Day 21. Pinkish brown spotting and very small amounts. Not appearing in my panties. Only when I wipe. Much different than period bleeding. Have been experiencing hot flashes, nausea and fatigue since spotting started. Today is Cycle Day 25 and still spotting a little. It’s more brown today. pregnancy test this morning shows negative. Help?