There are several reasons why a horse would buck. Pain (or discomfort) and training are the most common.
If her first buck was bareback, then start there. Some horses just don't really like to be ridden bareback, as it is uncomfortable for them - the best or the lightest rider doesn't change that for some sensitive horses.
I would still double-check the fit of your new saddle just in case.
Is bucking at the canter what is "out of character" or just bucking in general? How are you asking for the canter? Horses that are poorly/incorrectly set up to transition into the canter may buck; my horse bucks if I don't set her up correctly. If it is that case, I wouldn't correct her; give her a second chance. Try asking again by setting her up and give good, clear direction. Does she buck then?
What happened after her first buck? What did you do? What do you do when she bucks? Know your horse. You said that she is the type of horse that tends to try to take advantage of you. If you do nothing or fall off and simply get back on, she learns that bucking has no consequence. Even if the original bucks were cause by pain, discomfort, or confusion, some horses do transfer over that behavior even when they feel fine or aren't confused; horses do what works.
How often does she buck?
As for testing if she is sore, sometimes it's hard to tell. Gently palpate her back. If a horse is sore, they'll usually stiff up or raise/toss their head. If it's "not that bad" or if the horse is particularly stoic, that horse will not have such large clues. Some horses will simply give "that look" - you can see it in their eyes and you just know it's not comfortable for them. If you are unsure, you could ask a vet or a chiropractor to help you.
i posted this the same day it happened, it was the first time shes bucked with me (ridden her for at least a year and a half) and in general she doesn't seem like the type, i doubt its happened many times...
the first time she bucked i was all for putting it down to rider error (asked for the wrong lead and was off balance) but she did the same thing with her owner- who would not do the same.
If she didn't buck before, when you went into a canter, but since you got the new saddle she does, then the obvious first choice is to get the saddle fit evaluated.
palpate her back to see if she 'winces'. post photos here of the saddle on her bare back, for a look see on the fit, (though this won't be nearly as accurate as having a knowledgeable saddle person look at it)
the saddle fits- i dont think there is anything wrong with it as the first buck was before she had ever been ridden in it, ill check for soreness definatly
I also palpate the back to check for back soreness. If yes, then start looking for what's causing soreness. If not, look for soreness in other areas, bucking makes me think stifles, hocks...
maybe hocks, im not really sure how to check though?
anyway, im going up tommorow, hopefully she wont buck again but if she does ill post (and hopefully get a video)