Am I overreacting or should i feel slighted by this?

the girl i'm unofficially seeing and i text almost all day, off and on, when we can't talk over the phone. Just in the last three or four days she has been getting less and less responsive, just sending quick one or two word messages back, no punctuation, and i'm having to basically carry any conversation with her. Am i just suffering from ERP (early relationship paranoia) or do you think i should feel slighted? i've asked her about it and she says that nothing is wrong or has changed. What do i do? The rest of our time together is a blast, and she's totally normal then, its just over the phone...