Stopped latching at 3 months

My 3 month old has suddenly stopped latching this morning, she's been to the boob twice today for about 20 mins each time. She's getting bits from me but hasn't had a proper feed in the last 12 hours. She's still weeing and is happy, (She's just dozed off in her daddy's arms) but I'm falling apart because I don't know what to do. Can anyone help?