Is it normal that my girlfriend can't suck hard for too long?

Ok, here goes. I do well in punctuation and spelling and all that but I'm on my phone so I'm seeking forgiveness here. My girlfriend and I have been together and sexually active for six months and get tested regularly. We always try new and different things. She really likes to give me oral but after a while of sucking specifically on my pearl she says her bottom inner lip and where her tongue hits gets raw apparently from sucking. It's odd because I'm far from dry :). I also wonder if a factor of it is the time length of sucking. She can handle other items such as popsicles and sour candy. However, I think it's how long and how hard she's doing it. I'm actually hard to make cum so tops it's about 15 minutes if she sucks like that and 20 if for other things. So first I want to know is it normal and if someone can offer advice to help us or answer why it's happening. I'm really not interested in immature answers.