Exhausted & steadily putting on weight, loss of concentratio

In the last 6 months I have steadily put on 3 stone. I eat extremely healthily and go to the gym at least twice a week as well as other activities. I am also constantly exhausted. As soon as I wake up i'm still tired, and when it gets to the afternoon i feel as though i haven't slept for a week. This and the weight gain is really getting me down. I often suffer from a sore throat and lack of concentration. My doctor thought that I may have an under active thyroid as I have many of the symptoms plus my aunt and nan have it too. I have had blood tests though and the have come back clear. I really don't know what to do as I was once very over weight and managed through lots of hard work to get down from 14stone to 9 stone. Now my weight is steadily creeping back up and i'm terrified that I will be miserable and over weight again. Do you think that this is an under active thyroid or that it could be something else. My doctor just doesn't seem like she cares very much. Please help!