Hello,
Weight gain nowadays is a very common problem.
There could be medical,psycological or just lifestyle habits which could be a cause.
Let's talk about each one in brief.
Medical- the medical causes for weight gain could be PCOD (poly cystic ovarian disease) or
Thyroid related complaints.
Psychological- Increased amount of stress can in turn result in excessive eating a condition called Bulimia or alter your hormonal balance and in turn cause weight gain secondary to above two medical factor's.Its a cascading effect.
Lifestyle- Lack of exercise, lack of a proper diet plan,consumption of oily,fatty outside food in excess,sedentary lifestyle to name a few.
Rather than any medications or investigations what you need urgently is to consult a dietician and chalk out a diet plan according to your schedule. Also any form of exercise like brisk walking,aerobics,gym,yoga but under proper guidance by a qualified personnel will help you regain back in shape.
Try doing this for 3 months at a stretch if the weight still remains stagnant then you need to consult a doctor for investigations and remedial measures.
Regards.