iMessage sends messages but then marks them as "not delivered"

I have recently purchased a iPhone 5c 16gb within the past month and at the start i was using iMessage perfectly fine. I turned it off for 3 weeks as my plan was being reset and just yesterday i turned it back on to find that it wouldnt send any imessages. I updated the software and have reset all settings back to factory. I send a text and the recipient recieves it and on my phone it says 'delivered' but minutes later the message will change to 'not delivered' and have a red exclamation mark, even though the recipient has replied.

Please help as i have no idea what to do!

Thanks