Period Start Day? Help Please!

Jennifer • TTC baby #1 since 09/2016. 7 IUIs, 2 IVF egg retrievals, 3 embryo transfers, 2 miscarriages. Norah Josephine born 8/9/21!
Hi Glow fam! Need some assistance
Since going off of my BC, I don't know what to count as the beginning of my cycle. It's been similar to this the past few months but this cycle specifically, I saw a tiny bit of blood Sunday (Day 25) and Monday and nothing yesterday, and today (Day 28) I started with a regular medium to heavy flow. There was so little over these past three days it makes me think from what I've read that the beginning part could be blood left over from the last cycle. 
What would you count the start day as? The answer to this is going to determine whether I seek out progesterone therapy...mine was 9.2 ng/ml when I tested recently on Day 22 and it's normal but on the low end (they like to see > 15 I guess).
Thanks in advance!