Is it possible that the initial weight gain is due to an increase in muscle mass due to an increase in an exercise regime?

Don't look at it as weight gain when you are building muscle, a negative thought like that will sabotage your diet/exercise routine.
This is why it is important not to have the numbers on the scale be the basis as to how well you are doing on a diet. With exercise and muscle gain, the scale will rise but inches should be lost at the same time.
For example: I was on a low carb diet for 2.5 weeks and only lost 7 pounds (I was suppose to lose 14 or more) but what I notice was that I only lost 7 pounds but I lost about 2 dress sizes which would have been 20 pounds on the scale.
(I lost a lot of inches and weight through jogging daily but the scale increased due to muscle gain in my legs and else where)
Best of luck.
P.S. Also look for weight gain during PMS.

id say so.. i never thought of it that way..

not true. you will lose weight first then build muscle

when i joined weight watchers i actually gained during the first week despite following the programme religiously. the leader explained it by saying that any such change in eating habits causes the body to go into starvation mode and develop a bit more fat to keep it going. i then started losing weight the following week.

It is true that muscle weighs more than fat but it takes time to build muscle mass. If you have been dieting you should see a weight loss at first especially if you are exercising because you are eating fewer calories and burning more. If you are seeing a weight gain at the begining of a diet program you may be either retaining fluids, or the diet plan is not the best one for you.