There are several reasons your payment could have changed:  
1. Credit hours 
2. Retaking a class 
3. Book charges (These are estimated and subject to change) 
4. FAFSA requirements/eligibility. Please note that students must be at Half time (6+ credit hours) enrollment to be eligible for federal student loans and Pell Grants are paid on your school status.

Pell grant pay out

Full-time = 12+ credits

3/4 time = 9-11 credits

1/2 time = 6-8 credits

>1/2 time = 1-5 credits