I know that one well! Most dissipate by themselves but sometimes they don't - with the best of intentions medicos may be happy to leave it to sort itself out, and there is a caution about aspiration (taking out the fluid once the drain has gone) as it might introduce infection. But you are the one with the lump and the sloshing, and if it stays for too long it can encourage infection anyway as we all have potentially harmful bacteria in our bodies anyway. So see how you go and remind your medicos to keep if in mind. Best wishes.