Originally by: Herping yourDerp
if your in fleet ur getting a buff from fleet booster to shield HP. it's always happened but its usually a small amount.
What he said. But i'll elaborate a little bit more on this.
Due to eve's capacitor (and shield, these two share a lot of similarities) mechanic once you get an increase in max cap/shield amount (via module and/or skill/gang boost effect) it does not add the difference to the current cap/shield level, thus making it look like you've lost some.
For example, you have 1000 shield hitpoints out of 1000 hp max, then you online a module (in space) that adds 300 hitpoints to your shields (or cap, doesn't matter), you'll get your max shield hp increased to 1300 but your current shield hitpoints will remain at 1000. Thus, percentally, you'll see a drop in shields from 100% to approx 77%. Now, if you immediately offline the module, you'll return back to 1000 hp out of 1000 hp max.
Basically, this is waht's happening - you must be getting a fleet bonus that increases max shield hp value, which only works in the same system, thus you do not get it while docked (even if in the same system), so your shields will be charged to the maximum value w/o the bonus, and then maximum value will increase because of the boost when you undock, leaving you with a small dent on shield hitpoints (depending on the bonus size, it may get as high as +58% in some extremely rare cases - caldari titan's bonuses coupled with fleet bonus - but it will usually be under or equal to +15% if you're getting bonus from a character's skills only). And you can get bonuses from yourself too, if you're in appropriate position in fleet, there are more fleet members in system and you have the necessary skills.
p.s. armor works in different way, you get the difference in hp added to you when your max hp increases, which can be used to save ships if used at the right time. Or may result in your ship having negative armor hitpoints and exploding if done improperly. Seen that on multiple occassions. Same applies for structure, but you can only get structure hp boost from modules and your personal skills (skills do apply while docked tho)