I've done my first batch of beer from the Coopers craft beer kit - 1 week of fermentation + 2 weeks bottle conditioning. Over the weekend I decided to cool down and try one of the bottles. I noticed there was a weird "oily" top layer on the beer. It was really hard to take a photo where it's clearly seen, but I placed some arrows on the photos where to look. Is that normal? Or are those signs of infection?