My doctor said that because I lift weights and gained muscle that it kinda delayed my first period.

Okay, so your doctor is totally right. Intense exercise, especially if you're building a lot of muscle, can totally affect when your first period shows up.

Basically, your body needs enough energy and a certain amount of body fat to kick off your period. When you're working out super hard and building muscle, your body is using a lot of energy. Sometimes, this can delay puberty stuff like your first period because your body is like, "wait, we need all this energy for muscles right now!" It's not a bad thing, it's just how your body prioritizes things.

Your period will still come when your body is ready, even if it's a little later. Just keep an eye on how you're feeling and make sure you're eating enough good food to fuel your body and all that amazing muscle you're building. Being strong is so cool, and your period will figure out its own schedule.