My reputation points have been reset fully. I checked the activity tab and it says I lost 28 rep.
When I looked deeper I saw nothing that could have caused that. Does anyone have any idea why?
@Glorfindel is correct - you had an answer flagged as spam.
You linked to a survey-site offering "Free" Robux for completing surveys.
While you may not have known - those types of sites are typically very shady (if not outright scams), and should be avoided at all costs. Typically these sites are trying to steal accounts by offering something 'too good to be true', in order to trick people into handing over things like usernames, emails and passwords.
This was rightfully marked and deleted as spam.
As for your reputation points, well, unfortunately, you'll just have to work to build them up again. Remember that reputation is a measure of trust - having a post marked as spam is a fairly major negative in terms of how much the site trusts you.
All is not lost however - focus on writing detailed, high-quality posts, and you'll find that your reputation will improve quickly. For more help, check out these Help Center posts:
Looking at your reputation history, I can't help but conclude that you must have had a post flagged as spam or rude/abusive; if six users (or a ♦ moderator) flag a post as such, you'll lose 100 reputation. Your reputation tab should show which post it was. (Other ordinary users like me can't see those.)