It shouldn't be significant strength gains, but Clen activates mTOR. (the system that regulates our cell growth and metabolism). What you're experiencing is probably a small amount in relation to actual muscle fiber amassment, and more related to endurance, which in fact increases muscle cell endurance. In other words, your mind thinks you're stronger, but in fact you're just pushing yourself harder because of the effects. Either way, it's a good thing.