Frustrating and Discouraging

Are you tracking your calorie intake by measuring your food? Have you figured out your calorie goal for the day?

A lot of the weight loss in the first couple of weeks is water, especially if you switch from a high carb diet to a lower carb diet. Once your body gets used to the way of eating, the water doesn't fluctuate as much, so you may see apparent weight gain. It isn't fat, but then, the 5 lbs. lost the first week isn't fat either. Also, as gpanda said, exercise can cause muscles to retain water. I have learned not to weigh myself after a long hard workout.

How much have you 'slowly gained back'?

if you lost 15 lb in first month, that is quite a lot and some of it would be temporary water loss - which you then put back on because, temporary

So if you lost 15 lb and then re gained, say, 5lb, I would consider that just normal fluctuation or readjustment that happens once your body gets used to your new way of eating.
I would just check what you are doing and give it bit more time - you should see steady losses, but at slower rate than 15 lb per month, from here on.

If you are gaining, unless it's all muscle, it means you are in a calorie surplus and have to do a better job at tracking your calories. If you are doing that and still gaining then you have over estimated what you should be eating for your daily calories. It's really not more complicated than that.