Girls, Why might a girl leave me on read?

Okay so maybe I'm just overthinking it and it could be nothing but there's a girl I recently started talking to again after 4 months, We used to work together before she got recently fired and we were messaging each other on Monday & Tuesday which seemed perfectly fine but I messaged her earlier and she has seen the message but hasn't actually replied...

I know she doesn't need to reply straight away as she could be busy but realistically could there be any other reason? Maybe not interested?
Girls, Why might a girl leave me on read?
Post Opinion