Difference Amplifier Design
You need to have a negative supply voltage on the op-amp to get a negative voltage out of it. A rail-to-rail op-amp can, at best, approach the supply rails at the output. A non-rail-to-rail type might only get to +1.5V if the negative rail is 0V. 

So, the supply voltage must exceed the voltage you need out of the amplifier. If you need +/-4.9V, a +/-5V supply may do with a rail-to-rail output amplifier. If you have an LM324 it might only get to +3.5/-4.5V with a load resistor to ground.