Help! Periods irregular

So if someone could help me out. I got my first period back when my lo was 9 months in October which was about a week long. 8-14 In November got another period 12-16. But December I have not gotten a period. But have had very faint spotting on and off the last 2 weeks. Nothing that even shows on a pantiliner only when I wipe and it’s such a tiny amount can hardly even tell. But last night had sex and there was some blood but just spotting a little heavier but went away. I tested Thursday morning. Test was clearly negative. Could I be testing too early? I also still nurse my daughter so can periods come and go once they come back while nursing?