Whatsapp read receipt after enabling

Hi,

I have a question based on something that has happened me twice. I use to chat with a friend in Whatsapp who uses to have her read receipts (blue tick) disabled (I have it enabled always), so while I'm talking with her my messages appear with gray double ticks.

Then, sometimes, she enables read receipts (while we are not talking) and the ticks of the messages I previously sent her become blue.

So my question is what does it mean? This has to happen always? Did she open our conversation or after enabling read receipts all the messages become blue without clicking the conversation?

Thank you.

— modified on Aug 11, 2018, 7:22:31 PM