Perfectly natural side effect which most of us experience. Your natural hair loss cycle has been accelerated so the hair you’re losing you were going to lose anyway. It’s just happening more quickly than it usually does. The stress of reduced calories, the surgery & restricted diet, the hormonal flush & changes all contribute to the acceleration. Remember your body has more important things to use the food & nourishment you’re consuming for than protecting your hair at the moment. It usually only lasts about 3 months +/-.
Don’t worry, your new hair is still regrowing but at its usual rate. Any vitamins, supplements you take to try to stop your hair falling may only benefit your new growth. The hair on your head is actually dead. Can’t save that.
I know it can be frustrating & upsetting but the best thing you can do is just let it do what it’s doing. You don’t need the added stress of worrying about it. Some temporary hair loss is really unimportant compared to the weight loss & improvements to your health & welll being.
PS - Many of us cut our hair if it was long so it doesn’t look as noticeable & it takes less time for the new growth to catch up & be the same length. You can always grow it again if you want.