Having a large number of inquiries (someone pulling your report when you apply for credit/etc) can negatively affect your score. I believe the *general* guideline is 5 in a 6-month period. One exception to this is reports pulled within a 7-day time period, such is often done at car dealerships who submit your application to multiple different banks/finance companies. So if you have 5 reports pulled in a 7-day time period, it will not negatively affect your score.
There could be a hundred reasons why your credit score went down. Likely not due to your D2I, due to the fact that the credit agency likely doesn't have any information on your income. Credit scores do not analyze your income or your debt to income ratio.
However, it could be due to having a "sudden" large installment loan appear if you haven't had any sizeable installment loans in the past, or if you have other open installment loans with sizeable balances. Also because the loan is new, so the balance is large compared to your "original" loan amount. Likewise, if you recently obtained large student loans, that probably affected your score in a negative manor aswell. If you have more than 1 installment loan open, that will negatively affect your score aswell.
This is the fun thing about credit scores.....no one outside of the credit scoring agency actually knows how the scores are devired. They are very heavily guarded as the formula's are considered to be "trade secrets". The formula's themselves would blow your mind though......they are very, very, very advanced mathmatics and statistics involved.
Though they are very unreliable, IMHO. I've seen an 18yr old with nothing more than a single credit card open for 6 months with a score of 722, and a middle-aged man with reasonable currently open credit lines, a credit history dating back 10yrs and not a single late payment in his life with a score in the 650's.
