Have you checked your credit lately? Credit guidelines are pretty lenient for FHA loans. Some lenders will have additional guidelines but the minimum credit score is 580 for 3.5% down payment but most lenders would like to see a 620 or 640 for a 3.5% down payment.
To have a stronger credit score, it's important to pay all your bills on time and at the minimum have a credit card you use and at the very least, pay the minimum payment. Paying off your credit cards every month is ideal.
Also, having a mix of credit such as credit cards and personal/student/car loans is ideal.
Most lenders used the FICO score model. FICO calculates your credit score by: payment history (35%), amounts owed (30%), length of credit history (15%), new credit (10%) and credit mix (10%).
The most important aspect of your credit score is to pay you bills on time and secondarily to not to use over 30% of your available credit to carry a balance month to month.