You consider yourself responsible with credit: You pay your bills on time and you don’t rack up any major credit card debt. And since you’re looking to apply for a mortgage sometime in the near future, you’ve been actively monitoring your credit. But since you’ve checked last month, your score has gone down.

What gives? Why is your score acting so sensitive? Sheesh.

Time to play detective: The first step in cracking the credit mystery is understanding what factors affect your score, and by how much.

Here are the five factors that make up your score, according to FICO, a popular credit model:

• Payment history: 35 percent
• Amounts owed: 30 percent
• Length of credit history: 15 percent
• Credit mix: 10 percent
• New credit: 10 percent

Now, within each of those categories, there can be surprising nuances that may cause your scores to fluctuate. I asked credit and finance experts to explain some of these lesser-known reasons your credit score may be dropping. Here’s what they said:

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