iPhone 4s message bug

HI. SINCE I RESTARTED MY IPHONE THE OTHER NIGHT MY MESSAGE APP HAD A SOME KIND OF BUG. MY APP MESSAGE SHOWS THAT I HAVE AN UNREAD MESSAGES FROM MY GIRLFRIEND, AND WHEN I OPEN IT OUR WHOLE CONVERSATION IS GONE BUT THEN WHEN I GO BACK IT IS STILL THERE AND IT STILL SHOWS THAT I HAVE AN UNREAD MESSAGES. WHEN SHE TEXTS ME I CAN ONLY READ HER MESSAGE FROM THE NOTIFICATION CENTER, I TRIED DELETING OUR CONVERSATION BUT IT STILL DONT FIX THE PROBLEM. IT MAKES IT WORSE, WHEN I DELETED OUR CONVERSATION MY MESSAGE APP WILL BECOME LAG. THEN I WONT BE ABLE TO SEND A MESSAGE TO ANYONE ITS LIKE IVE BEEN BLOCKED BY USING THE MESSAGE APP. THEN I RESTARTED MY PHONE BUT THE NEW MESSAGE BUG WILL APPEAR AGAIN. PLEASE HELP!!