My mum in law also did not allow me to drink water for the first born. I also struggled tremendously with my first. But I drank for my second. Well, if you think that the relationship will be affected by not listening to her. You may try to do it either discreetly or, ask your mother in law to boil you some traditional drinks like the longan drink, or you may drink lots of milk as supplement, which is good for you and well...supplement your liquid intake.
I think your doc should have given you some pills to take to ease your bowel movements, if not, you may want to ask for it.
Continue to breastfeed by allowing your babe to suckle on you for 30mins on either 1 side or 2 sides. Take a break of about 2 hours before another 20-30mins suckle. dont't give up....listen to some music, watch your babe suckle or watch tv....so you dont't really just think about breast feeding. 4th day is still the beginning...have a little more patience, it helps. Usually, it takes a couple of weeks to see increase in milk flow. Your babe's stomach is only the size of a peanut now...so take heart.
I am no expert, but I hope I've helped a little by sharing some learning I gained from my 2 kids. It was a terrible struggle for me coz I did many wrong things too esp for my 1st so I can fully understand. Take heart, it is not impossible and it will only get better....I guess....that's motherhood.