Losing weight drastically. Pains occur?

Hi, 40 years Male here. I am noticing sudden changes in my weight. I kind of lost 6-7 Kgs in a span of 8 weeks, and continue to lose even after. I am currently at 67kgs and few months back was at 75Kgs. I also feel my bones are weak and fatigue. Situation is to gain just one kg of weight, I had to workout well, eat well and put in lots of efforts. But if I stop putting those efforts,I keep losing weight instantly and easily. This has been the case for me last few years and repeatedly happen. Also I always have aches n pains in my body, like knee joints, ankles, hip joints, or even elbows. I do regular workouts like few times a week and have been consistently doing this last few years. But still, I get joint pains and aches if I just stop doing it just for few days. Question is why do these pains occur? Is vitamin D deficiency an issue. Is it related to arthritis and how to diagnose n prevent it in early stages. Kindly advise and your expertise is needed.