Gosh this felt important so I'm writing a proper response
(For context I'm 24yrs old and got diagnosed in my early 20s. I've also been medicated since three years back and I assume you are as well.):
Yes. This is extremely common, both in your teens and into adulthood. My two cents is that the most important part is noticing that:
Yes, it does happen, and then find ways to work with it.
First step is to chart out your mood and energy levels.
You can use a mood tracker journal or an app with the same purpose. The main thing is it needs to be easy for you to use regularly and long term. (I use an app called daylio).
The point of this is to see when, why and how long you have these energy variations over time, so you can plan accordingly in the future.
Having a good energy day/week previously and using up too much energy, then running out very quickly the coming days, making your symptoms worse since you're tired.
Hormone imbalances, such as periods.
You're more stressed than usual.
There's not enough variation in your week and you're tired from sheer boredom.
Etc. Etc. (These are just the ones i experience).
Charting out your energy levels is also very important long term, because this aspect of adhd is sometimes misdiagnosed as bipolar disorder, so having these energy variations on the reccord can really help with future treatment. (Though there are ofc people who have both).
Then you need to find ways to mitigate the fallout of having less energy sometimes.
I noticed you're still in your teens, so your issues are going to be different from mine.
I have to hide money so I don't over spend, I have to prepare food so I don't suddenly go without, and I have to cancel meetings a lot to preserve energy.
As a teenager, though, I remember having a lot if trouble managing school and my own expectations.
Frankly, ideally, someone should help you with this but not everyone has that, so here are a few things I've picked up on:
1. Managing your schedule is very important, so being aware that a lot of ppl w adhd have time blindness will help tremendously with that:
Try to set up your week (visually) so that you value school/work as much as you do sleep and leasure activities.
Making sure you're well rested and not bored will help keep your energy levels stabile.
HowToADHD on youtube has a ton of good tips and resources. Highly recomend her channel.
2. Remember that Everyone gets tired. We just have brains that have a harder time managing the fallout of being tired.
It takes energy to mask your symptoms and it takes energy to mitigate fallout.
Starting on a task and ending a task takes energy as well, so scheduling in an hour or two between tasks (and leisure time/sleep) is pretty crusial so you don't do more than you're able.
3. Most importantly: be kind to yourself.
Thinking "why can't I do this today? I could do it yesterday" is sadly very comon. But frankly, being critical of yourself is much less usefull than acceptance.
Acceptance and knowlege is key in living with adhd I find. I hope this helps somewhat, and that it wasn't too all over the place.
(I focused a lot on scheduling bc i find it is essential to managing my energy, but you need to evaluate which aspects of life affects your energy levels the most).