Is a period that starts and then stops on the same day always implantation?

I am on cd 29 and I had a positive opk on day 14.

Today I thought I was starting my period as had a slight down discharge. This was 4 hours ago and I have had nothing since. When I have been to the toilet there has been nothing more in my knickers and nothing when I wipe.

My question is if your period looks like it's starting and then stops and starts is that always implantation bleeding?

Or could it still be the start of my period?

I had this happen in march and a few days later I had a chemical pregnancy.

I had sore breasts then but this time I have had no other symptoms only mild nausea.