Why is my girlfriend acting so strange?

ok so me and my girlfriend are in a long distance relationship and we love eachother alot and we are dating I'm 16 she's 15 and like today was a good morning and most of the day was good at 1 tho I was like I wish I was there she said me to then i like started kind of sweet talking and all she says is gtg that's it like out of the blue ahe says gtg and then like I say ok I love you and there's no reply till like 3 and she only says U2 and then I'm like what's wring she's like ok then I'm like what do you mean she says nothing then idk like I kept saying tell ms tell me I'm confused she's like alright then like she never does idk is she mad at me or what? Like this is out of the blue w didn't have a fight or anything night before we had a great phone convo ahe was worried about her mom like coming home cuz she was gunna get in trouble bit I calmed her down and idk today that happened please help what does this mean what can I do to fix it??