If you know you've made payments on time, and nothing was sent to collections, the fastest way to drop your score is to use too much of your credit line. Debt to total credit percentages can cause your score to ping pong month to month if you are constantly using more than 10-15% of your total credit. If you have just one credit card to build your credit, and it only has $500 for the limit, never charge more than $50-$75 per month without paying off the balance monthly.
If you check the "Credit Score" tab you can see the areas of your report that might need work. I might start there to see if a new account was opened, your utilization went up, or if there might be a mistake.
Hope that helps!