Getting negative voltage reading when negative voltage does not exist. Why?
There's a bit of ringing at about 70MHz. This is not unexpected unless your layout is really tight. A small amount of inductance in the wiring can cause this, in conjunction with stray capacitance. 

For example, suppose you were using a solderless breadboard with 5pF of stray capacitance (WAG) plus the probe capacitance of maybe 15pF. The inductance required to resonate at 70MHz would be only 260nH which could be created by a loop of wires.