As far as I know, weight change isn't a side effect of Accutane. Some people gain weight because they're making more of an effort to eat meals containing fat when they take the pills (the medication is fat soluable and works best if taken after a meal containing some level of fat).

Increased fats and maybe decreased output due to aches is a common thing. Some people find excercising more through the aches improves the aches and pains due to better circulation.