Talk to Gastroenterologist on Constipation While Weightloss

as salam alaikum am a 31 years old female and a mom of 3 kids Alhamdulilah my youngest is 15 months old and am breastfeeding my baby right now. i have been trying to lose from last year and have lost almost 15 kgs and now i have 10 kilos more to lose but the issue is i am constantly having hard stool even though i drink 5 litres of water everyday i have chia seeds on an empty stomach and i also have salads and high fibre roti every day i also workout and walk daily but my constipation is not going away ...am in extreme pain and feeling depressed.. which doctor should i see a gastroenterologist or a dietician? please guide me JazakAllah