I recently quit drinking diet coke and only drinking water. i feel bloated and gaining weight. is this due to quiting diet coke?

No: I applaud your decision to quit an unknown concoction(coke's secret formula) for something that has sustained us forever.I suspect another cause for the bloating and weight gain-unless the coke had a secret diuretic to get you to drink more coke.Keep up with the water and i expect you to get back to normal.

Caffeine : The reduction in caffeine can have something to do with the metabolic changes. Did your diet change otherwise? Continue to provide your body good nutrition options and if not improving consider an evaluation.

Diet Drinks: You note that you are only drinking water, that is great.But, i wonder if you are making up for desire for sweetness with food that may have more carbs, sugars or starches that reduced to sugar.Caffeine, itself is a natural diuretic in your body. If you no longer consume caffeine you could have some weight gain due to some fluid retention. A somewhat healthier option with some caffeine-green tea.