I'm 15 5'9.5 and weigh 151lb I recently used to weigh 145 but my clothes still fit and my waist is still 32.5 inches and it had been a few months since I've weight lifted I just started back so I don't see how it's muscle gain cause I gained 6lb in a week
There are two possibilities:
- You were just low on water content.
- You have put on muscle mass.
Possibility 1 is the most probable cause. Your body loses water and your weight fluctuates in a matter of hours. So, if you are dehydrated, then probably you lost a few pounds.
Possibility 2: You could have put on 2.7kg (6lbs) of muscle mass in a week, but only if you had worked out hardcore and took a lot of nutrition.
I weighed 85kg before and now I weigh 106kg and my clothes still fit. Don't take your clothes as a parameter to measure your gain/loss.