It looks as though my activity points started counting up from negative 100 sometime in March when I redeemed for a weanling. I was wondering why I had negative points 
I have been having the same problem, and posted a bug report about this in April: ⚠️ Incorrect Activity Points Balance
It looks like the problem started after redeeming activity points.
Odd things have been happening with mine, as well. After jumping back into the game in January, I haven’t tallied any points for entering races, even though I’ve got at least one horse running every Wednesday and Saturday. I’m also not sure how to account for the blank “+1” credits in my overview, or the fact that my purchase point credits aren’t listed there.
Unfortunately I believe points have been broken for over a year. Shanthi is aware, I just don’t think it’s a high priority versus other issues.
Makes sense! It certainly doesn’t bother me that much, I only just remembered Activity Points existed 