Loss of weight. You suggest me to take?

Due to repetitive work for almost 2 years, sitting in front of system without break for about 2-3hours in a stretch. From past one month I am not able to stand or bend for more than 5 mins. I need to flex myself after every 5mins. I am also an Archer, representing state as of now. I am worried that will affect my shooting career. What could be the cause? What are the preventive measures? How can I get back to my normal stamina of standing for 2 hours and shooting without sitting? Also I lost 4 kgs in last 2months due to heavy work ( which I hate to do @ Office). I want to regain my weight, stamina and should look healthy and fit n fine. Currently taking calcium n b complex tablets. I am also wanting to taking chord liver oil capsules. Which one you suggest me to take? Thank you so much for your patience and reading my case completely. Looking forward for your valuable information, suggestions and advices. Thank you Practo