You likely set your jump course BEFORE you picked up cargo or refueled. The Galaxy Map is literal to your ship's current mass. If you take on more weight, your previous course will no longer work, because your ship's mass would have increased.
Simple fix... Go back to the Galaxy Map and RE PLOT your next destination. As long as you are not taking on any additional weight, once you re-plot, your system jumps will be calculated based on your CURRENT weight/Mass.
Always a good idea to WAIT to plot your next destination until you are finished taking on missions etc. from the Bulletin Board or buying commodities.