Possibly some air in the line, but it can also be an indicator that your brake pads are getting thin. Most hydraulic brakes are self adjusting to compensate for wear, but if left for a while the pads can retract a bit further, meaning you need to pump them back up.
Check the pads first, but also check for any signs of leaking. The mineral oil used in Shimano brakes is quite stable; it doesn't absorb water or decompose, so unless there's a leak they should never need bleeding.
Just ride the bloody thing and enjoy yourself!