that may not be the reason. only the rating agency has the info to know why your score dropped.
paying large bills usually does not affect credit scores if paid on time. it could actually increase your score because credit utilization would go down. incurring large bill does.
heres some info from FICO
What goes into FICO® Scores?
FICO® Scores are calculated from the credit data in your credit report. This data is grouped into five categories; the chart below shows the relative importance of each category.
1. 35% - Payment history: Whether you've paid past credit accounts on time (10 years or longer is best and not every payment goes into your credit history)
2. 30% - Amounts owed: The amount of credit and loans you are using (rating agencies like to see that you are using less than 30% of your credit line
3. 15% - Length of credit history: How long you've had credit
4. 10% - New credit: Frequency of credit inquires and new account openings
5. 10% - Credit mix: The mix of your credit, retail accounts, installment loans, finance company accounts and mortgage loans
you should also know that you can have many different credit scores. different agencies use different models other than FICO
timing can affect scores which generally are updated only once a month
say your rating date with one agency is the 19th. on the 15th you incur a large bill substantially increasing your credit utilization. even if you pay it on the 20th, you going to take a hit on the 19th rating. next month assuming no more large bills or other events that can affect your score , it should go up
once a year for free you can get your credit report from the 3 major rating agencies. I do not know if they show credit scores on them.
