Your body has been through a lot of change over the last 6 weeks, some fluid retention and constipation can be relatively normal at this stage. Change in diet and your new daily vitamins can most certainly effect the bowels too and add to the water retention that you are experiencing. Are you keeping active? this can really help with stubborn bowels… have you tried prune juice? you may need to water this down tho to avoid any dumping. Are you getting in enough water?… this is a biggy and often forgotten about following surgery. The key is to have a drink by you and sip throughout the day, to enable you hit a good level of hydration. Are you getting enough fibre? Have you spoke to your dietitian lately? if not call them up and see can you arrange an appt to talk things through. They can go over your dietary input and give you advice regarding finding balance within your diet following your surgery. I would even advise you speak with your GP, get a little check over (they know your health history best) they may even want to run some bloods earlier than planned or prescribe you something to help ease the constipation.
How are things going with your weight loss?
Hope you are getting on ok xx