Thanks guys!

I was always checking the level after a ride and it was OK (1/2 between F and E). However last time I just turned the engine on, waited 5min, turned off and checked the level. It was at a minimum level. So maybe it was too short and the engine was too cold to increase the oil pressure. After a short ride the level was OK.

Do you change oil after winter? (mine was changed 5 months ago and I did about 250km on it till my winter break)