Hi, Everyone's body reacts differently to surgery, and it is just possible that your body is adjusting to the surgery you had. Almost everyone will lose weight and then stop to allow your body to readjust this is normal and affects people differently there is no hard and fast rule when you will stall but it is normal. Just think about it logically if your intake is less than what your body needs on a daily basis it will have to use reserves from within and consequently you will lose weight.
It is possible that one of the reasons for your headaches could well be lack of hydration. I find it difficult to drink as much as I should especially in the first weeks after surgery but it is getting easier as time goes on. Try to up your fluid uptake if you think you're not drinking as much as you should and see if that makes you feel better.
Above all, if in doubt do as others have suggested and see your GP if you have any concerns.
All the best.
AE.