Volkswagen Passat 2006: Oil pressure dropping after top up?

I was advised by a mechanic to top up the engine oil in my 2006 VW Passat. I bought Castrol Edge 5-30W as advised. I added the oil twice on the same day; the second time I topped it up the engine started loosing oil pressure soon after I started driving. I topped up more oil; it lost more pressure and started making a very bad sound. Any idea why it would relate to the oil?