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Hi, I have recently had my thermostat replaced after my car overheated when driving (the radiator and reserve tank were emptied and the heater was blowing cold air). When the thermostat was replaced the mechanic also checked the head gasket by a pressure test and concluded it was fine.

After I had the thermostat replaced, I took my car for a 5min drive and it began overheating but then went back down to normal fairly quickly. Once home and cooled down, I checked the radiator and reserve tank and saw that these were empty and approx a cup full of water/coolant was sitting underneath the car towards the back of the engine.

What could be wrong with it?

I don't want to be alarmist, but cooling system problems can sometimes be very difficult to diagnose. It's not uncommon for a pressure test to appear OK, only to find a little later (due to heat, expansion/contraction etc) that there actually is a problem with the head or head gasket. Leaks at the back of the engine can be from the head gasket, a heater hose, core plug etc (depending on the type of car).
However, are you sure the puddle is coolant, as it's very common to confuse condensation from the air conditioning system with a coolant leak in this area? Check to see if the puddle contains coolant additive (usually green in colour). If it is plain water, it may be just condensation.
Either way, if the radiator is empty, the coolant has obviously gone somewhere. You should get your mechanic to have another look at it. If the leak is external (as you suspect) a pressure test should easily find the problem. An internal leak (usually associated with head gasket failure) can be located with a tee-kay test. It is not advisable to drive the car in its current state.