Yes, sounds like teething.
Also, if she is approaching 6 months... this is also a growth spurt time and they get hungrier, and nurse more frequently, and need more in amount.
It also could be a combination of everything.
And yes, some babies get impatient about the let-down when nursing and they get frustrated, and thus, upset. My friend's son was like this, and my son as well. Just make sure, that adequate milk IS coming out... and she is getting enough intake. Nursing does take effort for the baby... the coordination of the mouth/tongue and the baby has to work at getting the milk flowing.
But also, if a baby is teething, is it harder for them to nurse too. It's common. But if it is teething, it will pass. It's just that the gums are uncomfortable and tender.
It should pass... teething will come and go. I've used the Hyland's teething tabs also. And my kids liked cold things...I'd freeze a dampened washcloth for them to chew on.