Lots of things could have caused a drop. Credit scores fluctuate for all kinds of reasons. There's no way for the people on this forum to be able to guess what might have happened without knowing all your accounts and what was going on with each one in great detail in the last 90 days.
One way to think about is that your credit score is that it is a bit analogous to your blood pressure. If you get your BP checked once a year when you get a physical, it might well be pretty much the same. But if you bought a high tech monitoring device that alerted you every time your BP changed minute by minute, you might find that it was jumping around a lot, depending on whether you just climbed the stairs, got in a fight with your ex, had a cup of coffee, etc.
Given that there's no way for us here to know, since we know almost nothing about your credit file, I will nevertheless make a guess based on the one thing you told us. You emphasized that you always paid your credit cards in full every month -- and you seemed to think that this would therefore cause your credit cards to have the same impact on your score no matter what month it was. This is a mistake. Today's credit scoring algorithms aren't able to see whether you paid your cards in full. So your wise financial practice of doing that has no impact on your score.
The way credit card balances work is this. At the end of every billing cycle, your card produces a statement. This is the Amount Owed. That amount is reported to the three credit bureaus. You then have maybe 25 days or so to pay your bill. You always paid yours in full, but FICO never sees that. (Though FICO will find out if you didn't make at least the minimum payment.) FICO just sees what your balance is at the moment the statement cuts.
If you are like most people, your credit card spending varies. One month it might be $700. Another month it might be $4400. (You bought a TV, say.) And so on. That causes your credit card utilization to jump around. CCU is a % that shows how much of your total credit card limits you are using. And your CCU has a huge impact on your score. If a person's CCU goes from 8% to 12% ( a small change) that could have an impact on your score.
Since you have decided you want to monitor your score closely, my advice is that you do some work learning how credit reports work and how credit scores work too. Right now you are lacking that framework of knowledge, so you are not going to understand what goes into a score and why yours might change. There are lots of good resources out on the web that could help you get that basic framework.
