I'll give you the shorter version of my analysis.
Pisces Ascendant and lagna lord Jupiter is well placed in Leo in the 6th house.
This in itself has so many problems.
Pisces is a Kapha constitution sign (mix of earth and water element) - so corpulence and weight gain are quite easy for this Ascendant. Jupiter in itself is another Kapha planet - so the problem gets a second dot.
Note the fact that Jupiter is placed in the 6th house of fighting diseases and weaknesses.
Jupiter in the 6th will cause sluggishness, digestion and assimilation is weak and slow.
Note the point that from 1998 - 2014 you are running the Jup Mahadasha - so we are all aligned to problems of weight gain.
Sun in general is well placed with Mars and Merc. Moon as the 5th lord of digestion, digestive organs is placed in the 12th house of Saturn. This causes weakness in that aspect as well.
I'm not an medical professional - so may not be able to diagnose your problem medically.
But you can do some remedies for the weight gain.
You'll need to keep away from Kapha foods - like pulpy fruits and vegetables like Bananas, soft dates, firm yoghurt, curds (dahi) etc., Also avoid sugary foods.
Add more activity (walking atleast a mile two times a day) to your daily regimen - Yes it would be difficult if you are already gained weight - but that is part of the remedy.
One thing should do is to perform about 12 rounds of Surya Namaskar at sunrise - this will jumpstart the metabolic activity in the mornings and help regulate the body well.
Adding some Yoga asanas like Shoulder stand and Halasana (plough pose) will also help in stimulating your thyroid gland to help improve metabolism as well. Improving your Metabolism will help win atleast half the battle of weight gain.
If you are inclined to get back in shape - You should learn some Pranayama techniques like Bhastrika and Kapala Bhati from a Qualified Master. Kapala Bhati is usually recommended for losing weight.
So lot of options to help you