FWIW I've been on it for 4 years and I find it is always the last half of the injection that is usually accompanied by a stinging pain about 4/10. I also find that some injections are painless and I might just have minor pain like any injection (flu shot etc) afterwards or I may feel nothing. Sometimes it is achy for the day or 2 afterwards and sometimes it is fine and I won't know unless I hit the injection point by accident or something similar. There are days where every time I sit wrong in the car etc I feel a bit of minor pain. It's really dependant on the injection technique, how relaxed my leg was, if I'd worked out that day or the day before etc.
My advice - lay down and try relax the leg, this usually makes for a good result for me. You're probably not gonna feel like a workout and as someone already said you're generally supposed to wait 24hrs after most injections prior to exercising. The pain is like a stinging, burning sensation in the butt cheek. I always warm the vial in my hands prior to administration as it makes it easier for the person injecting me. And I've found afterwards (some may frown on this) that I like to rub the area a little and walk around for a few mins.
Don't think of the pain bro - think of the reward. The changes and the affirmation of finally going through the puberty we missed

Although I will admit puberty in my late 20's/30s is a little different to being 14/15 haha!!!