Breastfeeding past 12 months and periods… advice please!!

I have been breastfeeding my 15 month old and my periods have been off due to a couple things, prob breastfeeding and my low functioning thyroid but I’ve been on meds for a couple months now and it’s trying to get back to normal (my periods). I noticed Wednesday I spotted like light reddish blood overnight and dried spots on my underwear and some in the toilet and some when I wiped. I haven’t spotted again since, thought it was my period starting honestly and now it’s Saturday. Nothing. So far, I have a negative pregnancy test since yesterday. I saw my OB for my routine pap and she told me to test in a couple weeks again if no period and if negative again I’ll come in for ultrasound and bloodwork. That shit scares me like what could it be ? I’m afraid of something bad. Is this normal and has anyone spotted like what I described??