Large puddle under car over night

[sarnomel99]

Can someone give me an idea what this might be. It might be a weird coincedence but my X5 was flithy, the wheels were black they were so dirty so yesterday I washed it and the wheels were perfectly clean again. So today I drove about 25 miles to a friends house and when I was leaving noticed a big puddle where my car was so I panicked because the history of my car has been nothing but trouble but anyway, it was black in color and somewhat greasy feeling without a smell. I checked my brake fluid, oil and transmission and they were all fine and none of them had the color or feel of the puddle. When I got home I opened my garage and there was a huge puddle from where my car was parked over night and obviously same color, lack of smell ect. What could it be? if it was just in my garage I would have thought that maybe it was just from washing all the filth off and it was just there from the washing which I did in the back of house. (I don't know why I though that, girl moment) anyway, can anyone help me out with this? I'm so tired of going to the dealership which seems to be the only place that can work on the car because of lack of whatever at other places. I would just like an idea of what it could be and if anyone thinks there is any correlation between me washing it and that happening right after because before that it wasn't leaking. The leak is coming from that plate thing under the car
Thanks