OP wants to shoot trap; 24" really doesn't work that well compared to at least 28; longer even better.
OP stated that she is new to trapshooting shooting, 5'1" tall, 90 pounds and her shoulder and arms hurt after one round of 25 .....
My thoughts? While 30"+ barrels and one oz. loads are generally accepted by most as "working better" for trap, I'm thinking that they won't work at all for the OP.
My daughter shot trap in 6th grade ..... she was shorter than 5' then ..... she used an ancient Remington Model 10 in 20 guage, with a 7/8 oz. load of 7 1/2 shot ..... she did not win any shoots, but did pretty well for a beginner, becoming a competent wingshot, and outshooting some kids her age with 1,000 dollar guns. The gun was light enough for her to handle, and more importantly, did not beat the hell out of her. I don't know how old the OP is but if she's young and going to grow any, dropping thousands of dollars on a shotgun is foolish...... better to learn the basics with a basic shotgun she can handle well.
I imagine her arms hurt due to fatigue, and her bruised shoulder was no doubt due to recoil, a problem compounded by heavy loads, poor stock fit and inexperience .....
Fatigue can be mitigated by toning the muscles- and just being a strong muscled person is not going to necessarily tone all the right muscles used in trapshooting ..... I found that out when I used to shoot with my daughter at trap practice..... I've always been a pretty good wingshot and would nail the first 10-15 clays or so, no problem ..... but then would inexplicably start missing ..... her coach said that was fatigue setting in.... unless you condition yourself to shoot 25 shot stands, it'll happen....
The recoil issue- light loads, a heavy gun. and most importantly, good technique and correct stock fit will mitigate recoil ..... but none of that matters unless the shooter can hold the gun and point it well for a full round...... an 8 1/4 pound gun that is over 4 feet long and has a length of pull such that the girl can't shoulder it properly and still reach the trigger just isn't going to work.