Can’t explain this spotting

Hi I am 35 years old and trying to conceive. In the last three months I have been spotting for one day ( few hrs in the day ) on the 19th day of my cycle, it comes w back pains, bloating and some cramps.

At first I thought it was implantation bleeding but then my period came spot on time. This has been happening for the last three months and I’m quite confused. Took a pregnancy test and they were all negative.

I have also never spotted in my life before these last three months. So I’m just wondering if nyone else has experienced this.

Thanks :)