Big belly syndrome is not because of her age but most likely due to the difference in hay quality. The hay you purchased a few months ago is not necessary going to be the same quality purchased at a later date. This condition can occur particularly when a hay is low in protein and very high in fiber. Adequate protein is not only essential to the horse’s body but is also essential to the bacteria that reside in the gut. The gut’s microorganisms are dependent on protein, and the micros are the only component of the digestive process that can break down the fiber portion of the diet.
Hay belly usually parallels an all long-stem forage diet and is often associated with pastured horses since they consume more volume compared to hay because pasture approximates 80% water and hay has roughly 10% water.
If you are feeding West Coast timothy then 3 flakes would approximate 17 – 18 pounds of hay. With the 5 scoops of timothy pellet – and I do not know the size of your scoop – conservatively that would be 5 – 6 lbs. A small coffee can of pellet hay is more than 1 lb so if your scoop is larger than a small coffee can the weight would be more. So 22 – 24 lbs of forage for a 1000 lb horse is 2.2 – 2.4% of their body weight which is the maximum forage for a light working horse. Another option is that you can use Timothy pellets to replace a portion of the timothy hay. Processed hays (pellets and cubes) should not be more than 50% of the total hay fed per day.
I suggest eliminating the hay pellet and replacing it with Integrity Lite. Integrity Lite is very low in starch and sugars and is available with or without molasses. Based on the information you provided, a 1,000+ lb horse in good health, dewormed 6 times/yr, with a light workload should be fed approximately 16 – 18 lbs of hay per day and 1.5 – 3 lbs of Integrity Lite.