its hard but its alway better to go even if its just to establish a pattern so that in future you know when your week might not be as good as you hoped you will know its still part of a downwards pattern and you wont get too discouraged.
Yes you should always go regardless of issue's like this so you can learn to understand you body better. If it's a gain you know why, so you dont have to worry about it as its not like you have cheated, Its still worth going as you might miss a great tip or recipe.
Yes! Go! You shouldn't miss out on class just because you think you may have gained - I was in the same position last week and was pleasantly surprised to find I had stayed the same (despite feeling like I had a football shoved down my jumper!)