How late does your baby sleep?

I've been questioning my almost 3 month old baby's schedule because she's been particularly fussy lately, she'll have crying fits where she'll shriek and scream unless she's held a certain way, and she will only nap about 30-40 minutes. She also doesn't eat well during the day, and will sometimes come off the breast screaming, even though I can hear that she's getting milk. Just last week she was a happy baby and would still nap for an hour or more, and had no problem feeding until she was full... I'm at my wits end and don't know what's wrong, so I also made a doctor's appointment.