Im 151cm in height, weight 39.5kg. past 1yr, ive been gaining weight on lower belly area. it doesnt feel soft when squeezed, implying it isnt completely fats. and it doesnt feel like a bloat too. no pain/constipation/diarrhoea. what should i do?

You are actually in the lower BMI spectrum so gaining weight is not the problem but the maldistibution of fat. The situation suggests to me that you are probably on high carbohydrate diet and stressful environment. You could improve your diet by increasing more protein, vegetables and fruits. You could also manage your stress better so the hormones could be modulated favorably for better physique.

Your BMI computes to 17.3 which is in the underweight category. Increasing muscular mass would be best with the understanding that body shape is genetically determined. Core exercises would be best for shaping abdominal muscles, current appearance may be caused by muscular weakness.

For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV vaccine. Wish you good health! Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines