First of all I would say that you need to get an exam and be tested for hyperthyroidism. This is when you metabolism is in overdrive and your thyroid is too active. That will cause weight loss or the inability to gain weight.
Then if that is normal. You can drink protein shakes in addition to your meals.
if i have hyperthyroidism can that be cure and if yes how?
Hyperthyroidism is treated with medication to decrease the amount of thyroxine your body is producing. Yes, most people will gain some weight after their levels become more normal.
So, you would need those tests to see if this is the cause for you and then depending on how high your levels are will be the type and amount of medication prescribed.
No, that is usually not the cause of those additional symptoms.
THose causes could be like Irritable Bowel Syndrome or anxiety driven.
The mild is most likely due to lactose intolerance. You can buy lactaid and take that before you have any dairy products.