Yeah, maybe you should go to the doc and explain the problem. He or she will probably recommend a series of tests. Thyroid function is a simple blood test and diabeities involves having a drink that tastes just like icing sugar. They give you a blood test before the drink, then you have to hang around for around 2 hours after the drink when they test you again to see how your body reacts to the glucose.
It will be best IMO to rule out any medical reason for your weight gain by being checked out before making any changes to your diet and exercise habits.
Even if it comes back negative, (and I hope it does), at least then you can move forward and make any changes necessary.
Look at these two links and see if you recognise any of the symptoms:
http://www.nhsdirect.nhs.uk/articles/ar ... icleID=206
http://www.nhsdirect.nhs.uk/articles/ar ... icleID=128
all the best.