Hi Kyle,
PMT function calculates the payment for a loan based on constant payments and a constant interest rate. I don't know what amount you have in cell G13, but if it is positive amount than PMT function will show negative result, if G13 is negative that you will have positive result. If you want to avoid negative amounts I suggest to use ABS(PMT(7%/12,3,G13)) function. You are mentioning that interest rate and months is in the input area so I guess you should refer to that instead of using fixed amount of 7%. But it's hard to check as you didn't share your file

Hope it helps at least a little.