Complaints grow legs the longer they are allowed to exist. If you ever find yourself asking, "How did this suddenly get to this situation?", then it's often because an earlier opportunity to recognise a complaint was missed.
Complaints can be and often are made in the form of 'softly presented' expressions of concern about something or someone. It is fairly common for these to be missed and consequently grow in seriousness.
A complainant could justifiably say, "I asked you to do something about this two months ago!" By this stage the legs of the complaint have grown quite a bit, and irrespective of what the original issue was the complainant may now hold an opinion on your skills, attitude, or even your parentage! It is also fairly likely they will have told a number of other people, either in person or, worse, on social media.
It's important to be alert to the possibility that what you're being told is a complaint. You can be sure by simply asking the person if they are making a complaint, especially if you are hearing phrases such as, "I am concerned...", "that doesn't satisfactorily answer my question", or "I am not happy about..." or numerous other possibilities.
We tend not to ask the question because we are concerned about the work involved if it is a complaint.
However, if you think it might be a complaint, don't hope it will go away. It won't!