There are various reasons why your FICO Score may have gone up or down. You can view the factors that go into your FICO Score here.
It's important to note that your FICO Score is calculated each time it's requested; either by you or a lender. And each time it's calculated it's taking into consideration the information that is on your credit report at that time. So, as the information on your credit report changes, your FICO Score can also change.
How much your FICO Score changes from time to time is driven by a variety of factors such as:
Your current credit profile - how you have managed your credit to date will affect how a particular action may impact your score. For example, new information on your credit report, such as opening a new credit account, is more likely to have a larger impact on someone with a limited credit history as compared to someone with a very full credit history.
The change being reported - the "degree" of change being reported will have an impact. For example, if someone who usually pays bills on-time continues to do so (a positive action) then there will likely be only a small impact on their score one month later. On the other hand, if this same person files for bankruptcy or misses a payment, then there will most likely be a substantial impact on their score one month later.
How quickly information is updated - there is sometimes a lag between when you perform an action (like paying off your credit card balance in full) and when it is reported by the creditor to the credit bureau. It's only when the credit bureau has the updated information that it will have an effect on your FICO Score.
Keep in mind:
Small changes in your score can be important if you're looking to obtain a certain FICO Score level or if you are striving to reach a certain lender's FICO Score "cutoff" (the point above which a lender would accept a new application for credit, but below which, the credit application would be denied)