Instructed to altitude above filed cruise level

[tgol 1]

When I was at my filed cruising altitude of FL270, the radar controller instructed me to climb to FL350. I followed the instructions and climbed to the assigned altitude. My question is, why did they instruct me to climb to a higher altitude although I was at my filed cruising altitude? My guess right now is that there was a lot of traffic since it was a Friday night over Germany. However, I did disconnect in the middle of the flight due to personal reasons, so I did not get any other instructions. This makes me wonder if the ATC would've instructed me to descend back to FL270. It would be greatly appreciated if someone could confirm this for me.